I bought this before I traveled to Bhutan in 2006 and it came in very handy. Though it is a hardback book it was essential while I was there. It is not a field guide and has few illustrations in it. Where it shines though is with range maps and altitudinal graphs.
There are several country maps and a lay out of how to use the book and the extensive altitudinal information laid out from 4000m to 0m with the months at the bottom and green markers to show records and also when the highest concentration of sightings occur. The range maps also cover three different colors for winter, all year and summer altitudinal ranges, essential in a country like Bhutan where many of the resident birds are altitudinal migrants. The range maps also have black markers for sightings to show the most common occurrence in that range.
The introduction to the book also covers the many different habitat zones and some of the more common species in those habitats.
The accounts cover world range and range within Bhutan with information about sightings and movements and breeding occurrence.
Though this is not a field guide it is essential for the distribution maps and altitude grids as the main guide for Bhutan, Birds of Bhutan does not have range maps, only Dzongkhag (province or county) letter references and unless you've memorized all the Dzongkhag names you will find yourself going back to the map at the beginning to find each one. The Birds of India does have range maps that include Bhutan but are quite small and cover the whole subcontinent and are not always on the same page as the plates. This is also true of the new Rasmussen Birds of South Asia though the maps are in three colors and are much better as well as being with the species in question they still cover the whole subcontinent.
If you are fortunate enough to go birding in Bhutan I highly recommend this book as I used it extensively while there for reference and once back to check notes on distribution. Though it was up to date in 2004 Bhutan still remains a pristine country with many protected areas so most of the information should still be relevant.
Birds of Bhutan (Helm Field Guides)
Birds of India: Pakistan, Nepal, Bangladesh, Bhutan, Sri Lanka, and the Maldives (Second Edition) (Princeton Field Guides)
Birds of South Asia: The Ripley Guide. Volumes 1 & 2