I'm embarrased to say :o but I tend to buy probably too many books because I love planning trips.
I have the Rough Guide (great detailed information), Cardogan for Northern Spain, Fodors (I love their walking itineraries in larger towns), Frommers, Insight, and Rick Steve's (good recommendations/insights but limited areas - only hits the "top" spots).
By reading several books I've found information that was not in any book but one. For example, on my recent trip to Galicia only the Cardogan guide mentioned the festival of San Telmo in Tuy that was going to coincide with my time there. With the help of fellow posters on this board, I was able to confirm the book was correct and the processional at this festival was one of the highlights of my trip. We would have never known about this festival, even after we had arrived, without this one book.
I usually don't bring the entire book with me.
Before I leave I tear up each book by city/region and only have to carry around the area I'm visiting each day. When I get home I use the books to label the pics for my scrapbook.
Re-reading this before I post, I realize I am a "planaholic".
Maybe I need a vacation.