for my Kindle Fire I have installed 'Comikat' which enables me to read .cbr files. Those of of you who read comics will find this very handy as the Fire does not have a comic book reader installed. Then i just Load in my CBR files via the Macro-USB cable and im good to go on my Comics.
as far as books i have yet to actually download one to the library but i doubt i will with very many books as i will be using my Fire more for playing Videos, reading Comics and Scripts for films which are reeadily available in .PDF format.
The Fact that unless im in a WIFI area i cant use the Internet is the downside (especially for when im riding the bus to and from work) but the ability to load and play my own videos and content is a great plan B for those non WIFI areas. if only the 8 (more like 6) gigs of memory where upgradeable to a larger count, but oh well.
Also, as for my video playing, I loaded some .AVI files onto the Fire but they did not play. The idea that Kindle didnt play the most wide used form of video file was a bit mind boggling to me, but it doesnt. So, After changing teh Kindle's setting to allow 3rd party software, I installed 'Mobo Player' and the lack of .AVI compatability was history.
So now, when im at the gym, on that Boring ass hour long walk on the Treadmill, I play An episode of Dexter, Sons Of Anarchy, Doctor Who, True Blood, The Walking Dead, The Sopranos, Boardwalk Empire, Game of Thrones, Californication, Oz or Torchwood (yeah, I dont see me reading a book on the Fire anytime soon either).