There are several excellent travel guides on Bhutan.
Lonely Planet Bhutan, Stan Armington.
The most comprehensive travel guide to Bhutan. Get the new third edition.
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. Updates are available online. Do visit Bart's Bhutan Treks website too.
Folded travel and road map. Scale 1:380,000.