I love Leanpub and buy a ton of books and contribute as much as I can there to works in development. The whole feedback process has 100% won me over! If I ever published a book (something I've fantasized about for years), it would be probably through Leanpub or self-published. I'd probably consider one of the smaller presses, but Leanpub would be my go-to. You guys do great work at Leanpub!