I have been living in this region for the last 6 years, and I have used several guides. I recently returned from a trekking trip in Nepal and I was not happy with this guide. For starters, it is very heavy - even though it is a field guide. The most irritating part, however, is the lack of detailed information about the birds' behaviour and habitats. Providing a description of their markings doesn't really help when I also have the picture there. If I am having a hard time identifying the bird with only its physical appearance, I want information about flight style, habitats and - especially for Nepal - altitude! Given that the guide covers so many countries, you will find pages of warblers and finches but without sufficient information to identify which one you have seen. My final point of frustration is that the distribution maps are rarely on the same page as the pictures and descriptions. Given that the area is really big, I can't really expect very detailed distribution info, but at least then provide altitude and habitat! But the maps are often several pages before or after the pictures, which means you have to keep the bird number in mind, the Plate number in mind, as well as the page number of the map - so that you can flip back and forth - really time-consuming and not at all practical if you are trekking (maybe easier if you can sit still, but then of course it isn't a 'Field Guide').
I was lucky enough to be travelling with someone else who had the Nepal guide (Field Guide to the Birds of Nepal), so I could quickly check when I was uncertain. That book is basically the Nepal version of the larger book with the same pictures, however, is much lighter with better descriptions and would have been a better guide for my trek. I would also recommend the Oxford Sri Lanka guide. There is also a Bhutan guide (again by the same authors with the same pictures), which is also very easy to use.