I have used a ton of ebook apps and have thoughts:<p>For me, the main thing that matters, particularly in a phone, is not wasting a ton of space. Margins do help readability. But the "margin" of your phone includes bezel, etc. Too-short lines of text drive me crazy and make me feel like my eyes are getting a workout.<p>Text-to-speech needs to be good. That means not speaking page and title numbers, working with the screen off, and turning pages properly with the screen off, such that it's easy to switch between reading the text and listening without losing your place. I have used apps that do text-to-speech but don't continue through the book, or only turn the page if the screen is kept on, etc. Also spoken words should be highlighted as they are spoken, if the screen is on.<p>Also on a small screen I much prefer ragged right to fully justified text. I also don't want a bunch of permanent on-screen chrome (tap in the center to reveal seems best); page-turning should be via a fast, simple "slide" animation; and the app needs to be able to override an ebook's font and spacing choices.<p>Cross-device syncing is nice as well. And this one is ridiculous but speed-reading modes are nice too, both through highlighting chunks of text and RSVP (one word or phrase at a time: Rapid Serial Visual Presentation).