A recent discussion on one of my writing-related email lists centered on when you are and aren't obligated to buy books written by your friends. The general consensus was that beyond your inner ring--critique partners, best friends, etc.--there is no such obligation.
That probably sounds weird to a non-writer. It certainly would have to me five or six years ago. I mean, if someone you know writes an actual published BOOK, how could you not buy it?
Thing is, once you start hanging out in writing circles, you start meeting authors. If you join organizations like RWA or HNS and go to writers' conferences, before you know it you know TONS of them. In 2001, I would've been in awe of anyone with a publishing contract. Now--well, it was pretty cool meeting Bernard Cornwell and Jennifer Crusie at the Surrey conference in October, and I could probably eke out a fangirlish "SQUEE!" for Naomi Novik, but authors no longer amaze me by their very existence.
By now I probably know at least 50 authors, maybe more. Some of those relationships are purely online (which doesn't make them any less real), and I don't know how many of them remember me as well as I remember them. (And, really, why should someone like Cornwell or Crusie remember talking to me the way I remember everything they said to me? To them, I'm one of the horde of aspiring writers, while to me they're What I Want To Be When I Grow Up.)
Anyway, if I bought every single book by every single author I know, I'd be broke, my bookshelves would overflow, and I'd have no space in my budget or on my shelves for interesting books by strangers. And the unfortunate fact is that just because I like a person doesn't mean I'll enjoy his or her book. Sometimes that's a bit awkward. Writers, by and large, are nice people, so it's always a bit painful to discover that X has no historical voice, Y's new book is boring, and that I don't like certain subgenres even when someone as wonderful as Z writes them. I'd much rather love all my friends' books, but that's not going to happen.
Fortunately, most authors I've met seem to realize that no book is for every reader, and I've never felt pressured to buy a book just because the author is one of my friends from my RWA chapter. I'll have to keep that in mind for when (I refuse to say "if") I'm published.