There are several excellent travel guides on Bhutan.

Lonely Planet Bhutan.
The most comprehensive travel guide to Bhutan.

Bhutan: Himalayan Mountain Kingdom, Françoise Pommaret
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.

Bhutan map
Folded travel and road map. Scale 1:250,000.