I’m going to make a couple of recommendations, but I do have a question - why are you looking for free/out of copyright books? These have a couple of issues that may get in the way of your primary goal of getting g better at reading and, I assume, learning about new subjects. I’m also going to make the assumption that we’re talking primarily about English language books, but note that you didn’t specify a set of topics.

Many of the books that you can find on, eg, Gutenberg, suffer from being poor or outdated translations. If you’re really looking to understand Marx’s Capital (to take an extreme example) I could not recommend a resource less than Gutenberg. It is atrocious. If you want to read Dickens or something, it is at best plain unflavored oatmeal. I’d like to suggest a couple of alternatives.

When I had zero income for a while and was simply burning through my savings to live from day to day but still needed to read and learn - both to feel human and to move on to my next phase in life - I found torrents of ebooks. Some of them were just crappy PDF scans where the pages were just images (I think my first Zizek book was like that), while others were available in or translatable to an e-book format. The ones I tended to grab (and this is 20+ years ago) were things like the entire collection of Oxford University Press books for a span of years, which would cover science, philosophy, literature, and so on. Each one was gigs in size, and I used an ebook program to catalog them.

The other option, depending on where you are or where you can manage to appear to be, is the public library system. This lets you borrow many books, and libraries aren’t all that strict (generally speaking) about validating addresses and such.

My suggestion overall is to read about reading, and in general to read more modern books. If you’re interested in Jack London, don’t just read Jack London. Read about how he fits into English literature by people who have dedicated their lives to the subject. Read about the world he inhabited, what his metaphors mean, and how he compares to writers in his genre who preceded and followed him.

Oscar Wilde is a darling author, but again you need the full literary context to really appreciate gay history and literature, you’ll want the additional history and context of his contemporaries, his historical conditions, and how his work influenced future authors.

While it’s easier to appreciate Sherlock Holmes than Shakespeare, you won’t get as much out of it out of either without a bit more digging.

I think all my suggestions are elsewhere in the thread, but I do want to point you to a couple sources for public domain books that are more readable than a lot of other sites.

https://standardebooks.org/ - Mostly cleans up and formats Project Gutenberg sources to make them more readable. Fixes typos, fixed formatting, properly uses ebook features. Very handy.

https://www.globalgreyebooks.com/index.html - Similarly cleans up and formats public domain sources to make them more readable, but has a lot of more obscure stuff. Especially good for weird bits of philosophy, theology, and esoteric stuff, but also just another good source to check if standard doesn't have what you're looking for.

I think someone else mentioned Anna's Archive, but that one's more than just public domain books. I've only used it a couple of times when I wanted to buy an ebook but the author/publisher set it to a price I wasn't okay with.

I really enjoyed reading Around The World in 80 Days, or whatever it's called.

I love Alice in wonderland, and heavily recommend the sequel, through the looking glass, which is also fantastic.

Two of my favorites

The count of Monte Cristo is also absolutely amazing.

Almost every Sherlock Holmes story.

HG Wells has something like 200 stories? They're all great.

I see you've already found Jack London. I remember enjoying White Fang a lot as a kid, though it's been a while since I read it so I'm not 100% certain how well it holds up.

Little Fuzzy by H. Beam Piper is a delight to read!

https://www.gutenberg.org/ebooks/18137

If you like it though, the next two are not public domain.

This one is under a Creative Commons non commercial license and available for free. Plus it’s one of my top 10 books of all time: https://en.wikipedia.org/wiki/Down_and_Out_in_the_Magic_Kingdom

I am a big fan of Leo Tolstoy. His biggest works are War and Peace and Anna Karenina, but those are both quite large, so if you want to know a bit about his style i would suggest some of his shorter works. Of the shorter works I have read I really loved "The Death Of Ivan Ilyich", "Hadji Murad" and "Master And Man".

I also really like Jane Austen, you can't really go wrong with her. Her novels aren't that difficult either; they were the first novels in English (my second language) that I really enjoyed reading and they significantly improved my English.

I read the classics a lot when I was younger.These are ones I've read and enjoyed:

Anna Karenina, The Great Gatsby, Twelve Years a Slave, Frankenstein, Moby Dick, Little Women, The Importance of Being Earnest, A Tale Of Two Cities, Metamorphosis, Crime and Punishment, War and Peace, Dracula, Thus Spake Zarathustra,Beyond Good and Evil, Little Women, Don Quixote, Meditations, Gulliver's Travels, War of the Worlds

There are plenty more lists out there of good public domain books, but many are not my cup of tea.

The Road by Jack London, Twenty Thousand Leagues Under the Seas by Jules Verne. In fact, I have a load of DRM-free ebooks I can just give you if you want.

The Phantom of the Opera by Gaston Leroux is a fun read.

You'll probably know Flatland.

For the public domain, I think Les Misérables by Victor Hugo is a great read.

However I must ask, why limit yourself purely to the public domain? Do you not have access to a library? They are incredible places. And there are also some great apps available like Libby where you can checkout ebooks and audio books from anywhere with internet.