Although we're not generally ones for hauling guidbooks along with us on trips, we did it for our first trip to Bali and were glad we did. The vast array of places and place names, stuck together as they are in the South Bali area, makes a guidebook really useful when trying to answer the inevitable question of "what do we do now?"
The Lonely Planet and Rough Guides are of course safe bets, but we like the Eyewitness guides. They're especially good choices for first-time visitors. The guides are heavily illustrated, which helps you decide where to go and what you want to see before you get there.