Amazon is the best place to browse and shop for ebooks, IMO. This is where the majority of my ebooks come from. I got a few Nook books from Barnes and Noble and from Apple's iBooks. But if you do so, you're going to need to strip the DRM and convert to Amazon's format to read them on your Kindle. So if you don't want to go through all that, I suggest you stick with Amazon's Kindle Store since the books are "Kindle-ready". Also you may have heard of Project Gutenberg? It's the oldest digital library.
For subscriptions on Kindle books, check out Amazon Delivers. There's a Kindle category there where you can choose from a number of Kindle subscriptions. For instance if you subscribe to the "Kindle Daily Deals" you'll receive daily emails about Kindle book deals from different genres.
Image