The trouble with ebook lock-in is that if your whole library is on the Kindle, you won't want to buy a book from another store. This is silly. We'll show you how to share books between libraries in a minute, but first you need books to start with!
Ebook search site Luzme is a handy tool for comparing book pricing among various stores. In addition to showing you prices for a given title across several services (including Kindle, Google Play, iTunes, Nook, Sony, and others), it also shows you how the price has changed over time so you can see if it's at a particular low point or if it it's likely to fluctuate at all.