Get the logistical and other details before you go - it just makes the trip more meaningful. Both Frommer and Fodor provide valuable information. I use those for logistics, and then I get the Dorling-Kindersley books for the cultural and historical information, plus the great detailed photos and 3D color line drawings of the sights and areas. Another wonderful book, Zion & Bryce Canyon National Parks, is written by Sarah Benson, Lisa Dunford and Carolyn McCarthy, who provide wonderful conversations about history, geology, culture, and more.
Soaking up this valuable information always enhances our trips so that we can deeply understand and appreciate where we are, who we are meeting, and what we are seeing.