I used to do that. I actually started with an iPod touch back in the day, then switched to an iPad/iPhone combo when iPads and the iPhone 4 came out. I miss their books app, which I still feel is the best e-reader GUI. Their books seamlessly worked between devices too, I loved reading at home with the iPad and then I could just use my phone on the train when commuting, etc.

Problem is, Apple is the greasiest company in tech, and their customer service is probably the worst in the industry, and their cost to value ratio is also probably the worst.

I had a Google Nexus 9 tablet and I've tried using my android phones off and on, but the Google Books app just doesn't do it for me.

A couple of years ago I wanted something a bit waterproof, so I could read on my floatie while in the pool. I ended up with a Kobo Libra Color, which I actually quite like. It's an extra device to cart around but it's nice and thin, you don't notice it in a bag. I also don't commute nearly as much nowadays, so it's less of a factor. It's still not the apple app, but you can still side load books and it's definitely easier on the eyes over the longer term.