There are lots of guidebooks about Hawaii; here are a few of the ones we find most useful.
Best single guidebook to all of Hawaii. Lots of information in a single (large) volume:
Best single-island guidebooks for Hawaii. Detailed maps, descriptions and directions to the best beaches, and good reviews of hotels and restaurants (both big and small). We've found them especially good at providing directions and information about lesser-known sights and beaches that other guidebooks omit.
The following guidebooks are also excellent ones. These focus on outdoor activities - particularly hikes and beaches.