I am a voracious reader of reference books - design, illustration, and - - marketing, yes, marketing books. I was bitten by the PR/marketing bug three years ago when I officially started my own business and now my well of fascination/interest with the subject is never full. Here are a few books (and authors) whose PR / marketing books have been indispensable reading for me. (I have read and reread them with highlighter in hand..)
Guerilla P.R. by Michael Levine
Really great book, though it's a little outdated, but still worth reading and having in the library
Off-the-wall Marketing Ideas by Nancy Michaels & Debbi J. Karpowicz
Great ideas and extremely easy to digest. You can pull this out anywhere and read a bit no matter how little time you have.
Full Frontal P.R. by Richard Laemer
A little chunkier to get through, but very worthwhile.
Just about everything I've read from Jay Conrad Levinson and Seth Godin
including (these are from Godin)
Free Prize Inside
The Big Red Fez - This one is a little dated-looking since it is all about web site design and organization, but it is still very worthwhile to check out because much of the same rules still apply today, PLUS the overriding philosophy DEFINITELY applies (i.e. "get out of your own way" in connecting your customers with your offerings)
Unleashing the Ideavirus
...Another must-read which is not a "marketing book" per se but nonetheless is a user's manual of sorts on the philosophy of trends, and from a marketing standpoint is a must-read:
The Tipping Point by Malcolm Gladwell