Also, having a Kindle don’t forbides you from buying books (quite the opposite, you may want to buy more), either from amazon or other editorials that make digital versions of their goods and can send to Kindle (and if not, calibre can convert their formats to Kindle or the reader you want).
Thiner, lighter, than most paper books, plus you can store plenty of books on it, is not “i just finish this book and tomorrow will start the next one” as you are not carrying both at the same time, you just need one kindle to rule them all
Plenty of great public domain and free books (and in general online reading material, like articles, blog posts, magazines, etc). PDF reading is a feature too, but my experience with the Kindle 3 was not very good with at least some of them because text quality vs page size. In the Kindle Paperwhite 2015 (300 ppi) the extra resolution solves pretty well that problem.