i use a Kobo Clara HD. It runs linux out of the box, the system memory is on a removable SD card inside the case, and the user account is defined in an SQLite database on disk. If you add an empty user account to the database, it removes the "create account" screen and disables any Kobo online services. Then you can install KOReader and upload files over USB as everyone else has said.

TLDR

Probably anything will serve, but I would recommend either a Kobo or a Pocketbook for they seem more open (or maybe Onyx, I don't know this one!). I only ever dabbled with two devices though so I won't claim I know a lot about the subject. You want to read books with ideally the epub format.

How to read pirated books

Download the book, connect the e-reader via USB (maybe can be connected in other ways but wtv), copy and paste file, done. (Or use Calibre to manage your library and not do this "hard" step manually).

When you open your device, you will find the book there and you can read it. For any practical purposes this is the most straight forward and means it may not matter that much what you choose. This is to say, provided you find the books you want to read somewhere online, you shouldn't have troubles reading it in an e-reader. This works in all e-readers as far as I know.

E-book formats

Kindles used to support less formats, specifically not epub which is a book format. They did support mobi which was a good alternative. I read a while ago that they were going to start supporting epub but no idea if it's the case or not.

Epub is the open format that I go for and probably the best and most compatible. You can edit epubs by modifying HTML and CSS actually, if you know how to open them (hint: Calibre). This, to say, I considered supported book formats to be relevant but maybe they're not a big problem either.

Edit: Looking at this, it seems that Kindles still don't support epub. They do support mobi though and like @boredsquirrel@slrpnk.net said, we can convert epub to mobi fairly easily, so it's not a deal breaker, just one restriction that I would personally not want to bother with.

Openness/customization (KOReader)

KOReader is a third-party software that you can install on e-readers to provide a more customizable experience. I'm not going to try to convince you it's good or not, nor that you should install it. I installed it only a couple of months ago.

I'm mentioning it first of because you may be interested but also to point out at its installation method in Kindle, Kobo and Pocketbook. (They also have installation method for Android which I'll leave out because Android, and Cervantes and reMarkable which I'll leave out because I don't even know what these are).

Kindle has to be jailbroken, Kobo seems fairly straightforward with maybe a minor annoyance step and Pocketbook seems straightforward. I have a very old Kindle that I don't know if it's supported (didn't check, cba), and a Pocketbook Touch HD 3. I delayed trying KOReader for a long time because I assumed that it would replace my Pocketbook's stock software and it serves my purpose well, but on a second glance more recently I realized that the installation steps (copy + paste) didn't override a single file in my device and KOReader lives side by side as an app in my reader.

For that reason I would suggest either Kobo or Pocketreader.

I forgot about Onyx which @neshura@bookwormstory.social mentioned - I only became aware about these recently. I suggest you take a look into this as well, it looks decent but I don't know anything to comment.

Models

I don't have specific models in mind.

For my more recent Pocketbook purchase, I wanted something closer to 6-7 inches, 10 is too much and I wanted to have physical buttons to turn pages and not just a touch screen. The button requirement limited my choices heavily. Looking back in hindsight I don't use these buttons, I don't need them, but my first device had no touchscreen so... :).

I didn't like the position of these buttons on the Kobo Libra 2 or whatever (and maybe it was also too big?), and those were pretty much my concerns.

My model of the Pocketbook was already an "old" model when I got it. My only concern mildly related to piracy was: Does it support epub? But that's not a restriction - you can read other formats.

Kobo , hands down.

If you JUST want to read pirated ebooks? Kobo is probably the best bang for your buck. But you can also pretty easily sideload ebooks to any kindle via the email interface (which I believe Calibre can utilize).

That said? I have a mix of ebooks I got from legal and less than legal sources. And some of those legal sources include amazon kindle because the prices are REALLY good.

So I like my Onyx Boox. Yeah... it is jank as hell and it allegedly comes with a free 5g modem so be wary of what personal info you put on there. But it works well as I can use the kindle app (which also syncs with my phone) for amazon stuff and the native ereader for any epub files. And because I use a webdav to sync my notes, grabbing new books is as simple as remembering to scp a folder to my nextcloud periodically.

I've been using Kobo for a decade, never had a problem loading any type of file.

The first one I got was crap; the casing fell to bits. They sorted their shit out in the last few years and now I have a Clara E and it's honestly fantastic

I love my Kobo Libra 2

Kobo works fantastic.

If you have the ability, set up calibre and calibre-web and you can configure your Kobo to use your ebook library as the “store”.

Kobo also has at least one plugin/mod that replaces the whole reading UI with one with more features. Check out KOReader for that.

Apart from that, though, it makes little difference what ebook you get. If it allows you to load your own ebook files on manually (afaik they all do), you can do whatever you want.

Onyx uses Android for their OS as such there are pretty much no restrictions on book formats

I'm not sure I understand your question. I read pirated (DRM free) ebooks since 5-6 years on my Kindle Paperwhite and never had any trouble, even been using their ebook via email function without problems. I think they don't care at all.

I think all non-Amazon readers support the full variety of ebook formats regardless of the source

you can pirate on a kindle it's just more annoying to do

Kobo is the go-to for bang for buck readers that don't care where your files are from and have good format support. got my dad a libra 2 and it's great, especially with the physical page turn buttons. the default reader opens most files just fine, but you can also put KoReader on them for more functionality without too much hassle.

Personally I use an older Boox Note 3 which is easier since it runs android, but is massive overkill to be used as just a reader, i use it as my main tablet and a notepad/sketchpad.

Kindle paper works well, since you can just sync books into it with calibre or other software. They're also amazingly tough to break in my experience.

This is not a suggestion, it's probably fairly stupid, but it's what I've been using.

I've been using a convertible ThinkPad L390 Yoga as eBook reader as well. I never considered a 2-in-1 laptop, but it was cheap and I heard the Yoga versions have better colors (display). I thought I'd never actually use it in tablet mode, that my touchscreen would be unused, free of smudges. Hell, I didn't know what I was missing, it's awesome.
I've been using it to read eBooks, in portrait orientation as a tablet.

Software wise, Arch Linux (btw), KDE Plasma 6, Arianna eBook program.
Not optimal to be honest, Plasma 6 has some annoying bugs, and Arianna is broken as of recently. I suspect some depency issue, but anyway, for the time being I use the Flatpak Arianna package.

But I do like the experience. If I need to check some word in dictionary I can do it on the same device. Plasma 6 has touchscreen gestures, for example I use sliding from right to switch between windows. So, Arianna and Firefox with Wiktionary open at once, reading the book, unknown word, long press it, copy, slide from right, Firefox window, paste into Wiktionary, boom!
And to save extra power I use Bluetooth for network connection rather than WiFi. 1Mbps is plenty for dictionary searches.

Oh, important to me, when turned around there's a deactivated keyboard on the other side that I can fidget with while reading. I feel like it helps keep me from getting distracted by something else. Just mashing the keys with my right hand fingers and clicking the trackpad with left.

Disadvantages of this:
Hardware wise, it's a 1.5kg 13.3 inch eBook, so... perhaps not your glass of water (I don't drink coffee ¯⁠\⁠_⁠(⁠ツ⁠)⁠_⁠/⁠¯).

(No need to read further, this is specific to software I run on it.)

Software wise, well, you can choose different software, but bugs. Visual glitches like the taskbar switching to floating when using virtual keyboard or the window occasionally staying retracted from where the keyboard was (fixed by toggling affected window out and back into fullscreen) are okay.
What's worse, inactive window translucency can get stuck, i.e.: if the window gets stuck translucent even in foreground, and you close it, it's now permanently on screen as ghost window and you'll have to log out and log in again.
Worst, toggling Bluetooth (usually when done quickly after log in) may crash the system partially. The GUI completely freezes, tty works, but reboot won't fully work. It will get stuck mid-way, so I recommend logging in as root, enabling magic sysrq (echo 1 > /proc/sys/kernel/sysrq), issuing reboot, let it freeze, then Alt+SysRq+REISUO (one by one while holding down Alt and SysRq keys) to shutdown.

(Bluetooth service cannot be stopped or killed, nor plasmashell)

P.S.: Use Wayland with touchscreens. X11 has no touchscreen support, it just emulates a mouse pointer which is suboptimal.

Anything that supports EPUB, AZW3 or MOBI. So basically anything.

And should have like 8GB of storage at least.

I used a Kindle Touch, just didnt connect to the internet and used Calibre to convert EPUB to MOBI without issues.

No idea about piracy …, but Kobo is definitely your go-to for any file format.

I use Pocketbook. It opens just about anything - epub, mobi, pdf, pdb, and many more formats. Just get a book anywhere and copy it via USB. Or send it as an email attachment to your special address and it will download automatically. You can even replace the reading app with another relatively easily, if you want.

https://koreader.rocks/

I use a Kobo Libra 2. It was super easy to install KOReader and it just works.

I'm genuinely baffled every time I see people suggest KOReader.

It has the worst library navigation I've ever seen.