The nice thing about Play Books is that, like Play Music, you can upload a library of content you already have. Down side is that Play Books may not be the simplest cross platform choice (there's no ModernUI or Windows Phone or iOS version - those users can work through the web if that's something that they understand how to do in the first place).
Kindle works on pretty much everything, but it's a GIGANTIC PITA to extract content if you want to read a purchased title outside the Kindle app (for example, if you want to read using Aldiko or Moon+ reader).
Philosophically, I wouldn't want to tell someone "just buy from Amazon and forget other things exist" but there's no simple path for non-techies to juggle content between Amazon and anything else and it really is the biggest ebook marketplace. It's very similar to the lock-in that many people think Apple has with digital music sales, though at least Amazon is nice enough to make their software work on fifth-rate platforms like WebOS and iOS.
