Friendless Fans?
Few authors have a Maureen Dowd/Bill O'Reilly-style publicity machine. So the biggest obstacle to most book sales is simple ignorance. People who might like a book simply don't know it exists. The most likely way they'll find out is if a friend reads the book and recommends it. Hence the importance of buzz.
Lately, I've been fretting over The Substance of Style's respectable but unspectacular sales (roughly 18,000 copies in hardback, now out of print, and 12,000 copies so far in paperback). One problem seems to be that, while the book has enthusiastic fans, it has gotten minimal word of mouth. Why? Professor Postrel's cheery explanation: "The people who like your stuff don't have any friends."