PostGIS in Action is a great book
PostGIS in Action is one of the best tech books I have ever read. In addition to being a thorough guide inside and out to PostGIS it is also full of tips and tricks for the Postgres database itself.
My only reservation with the book is that now might not be the best time to pick up a copy. With the big additions PostGIS 2.0 will bring soon I would be hesitant to pick up a copy until it has been updated for the new version.