@intwtl, if you're already heaviliy invested in Amazon books then the Fire could be right for you, but the Nexus 7 runs an open Android platform onto which you can install Kindle Reader and be able to read Amazon books. But the N7's openness also means you can have access to anything on the Google Play store, including Calibre Companion, and you can read books and watch movies etc. that aren't from Amazon. Make no mistake, any Kindle is pure and simple an Amazon consumption device. If you get all your media from Amazon, that's all well and good, but if you want to be open to getting media from other sources you'd be better off with an N7. (The Nook tablets are in the same boat as the Fire tablets; they're "Android" but they're severely locked down, although still more open than the Fires.)
If you're considering a dedicated reader I can add to the Doctor's Sony recommendation with a wholehearted endorsement of Nook Simple Touch, with or without Glowlight. It connects as a disk drive and Calibre handles it flawlessly. And you can get in-store support for any B&N Nook product. I had the non-Glowlight model and passed it on to a family member and got the Glowlight model and love it. As passionate as I am about my N7, the Nook is much lighter and thus easier on the hand/arm over extended periods, and the ergonomics are well designed for reading. If you have a B&N anywhere near you, I'd go check it out before purchasing anything.