Kindle... But it broke... I have my books on an Ipad and on PC as well but I can't take them everywhere I go because they are heavy and fragile
I have a tablet made by Acer has Android. So I have the Amazon app for that starting to read so really good books.
Nook Simple Touch, rooted with touchnooter, with Kindle app, Nook Color with N2A microSD (turns it into a full Android tablet), iPad 1, HTC Thunderbolt, Kindle and Nook apps, and HP Touchpad, with Kindle and pReader...yeah, I'm a geek...
I noticed Calibre was mentioned but spelled wrong above. It is a great converter and library manager...and a free download.