I like buying books from Amazon.  They have really done a great job of making book buying friction-free.  But I love buying book from the Pragmatic Programmers even more.  Why?

1. All their books are available in both paper and ebook formats.  Recently I've been trying to cut down on actual physical books and have taken to only buying ebooks.  It's great that all their books are available in ebook formats.
2. Note that in #1 I said formats (plural).  Their ebooks are available as pdf, epub, and mobi which means I can read them on my computers, iPhone, or Kindle.  This is huge for me.
3. Book availability in beta.
4. I like the subjects of many of their books.  They have books not only on various technologies but also on management, self-development, testing, etc.