If you are only doing books then the kindle and nook work great…but they are only for either Amazon(kindle) or Barnes and Noble(the nooks).  I would recommend you go with a full function tablet if you want to do anything more.  You can get amazon apps for all things Amazon and Barnes and noble apps for those vendors as well.  You also then have the Android market(something the Kindles and Nooks do NOT have access to) to flesh out the rest of whatever you want to do.  Right now the Samsung galaxy tab, Motorola xoom, and the Asus transformer (transformer prime) are the front runners.