There are several excellent travel guides on Bhutan.
Lonely Planet Bhutan.
The most comprehensive travel guide to Bhutan.
Bhutan: Himalayan Mountain Kingdom,
An excellent travel guide with a lot of cultural information.
Bhutan: A Trekker's Guide, Bart Jordans
All you ever wanted to know about trekking in Bhutan. A must if you are going on a trek in Bhutan.
Folded travel and road map. Scale 1:250,000.