This guide book (2007) is a really good planning tool for a trip to Cuba. It has lots of photos and great schematic drawings of important buildings and plazas.
If you are taking a tour package where you are not having to choose accomodation yourself or deal with transport logistics, I would probably take it with me. However, for the traveller who is making all their own decisions and reservations, I would take either Lonely Planet (new edition Fall 09) or the Moon Handbook (06). using the DK just for planning then leaving it at home. DK has minimalhotel recommendations - just the chain hotels, and sketchy restaurant selections. The Moon Handbook has really extensive background information on Cuba, but weighs almost twice as much as Lonely Planet.