I usually bring a map and guidebooks to most places i go. And in between times I browse the bookshops for other destinations. Including some second hand shops for antique guises or other material.
What material do other posters use?
Do you use the same publishers for different destinations? How much material do you bring out or pick up when you get there?

Also, I half read an article from one of the English Sunday papers about a reduction in Travel Guide sales. I couldn't find it with a search but here's a couple of sites with a similar theme.

A question and replies to a similar question

LA Times article on launch of TripAdvisor

discounted books