More on "sideloading" Android apps to the Kindle Fire.
Aldiko ereader is listed as being incompatile with the Fire on the Amazon app store but I just sideloaded it and it works fine. I just downloaded a couple of public domain books. I have read that the Kobo reader also works well. How to Sideload Apps On The Kindle Fire
What is nice about the Fire is that Amazon has left a side door in the device. Under Device settings they have a button "Allow Installation of Applications from Unknown Sources." As a result there is really no "hacking" of the device involved or necessary. Pretty slick.