A good field guide is just as important as a good pair of binoculars. Broadly speaking, field guides now fall into two categories: the traditional bound books and (this year!) the mobile phone apps.
Bound (traditional books) Field Guides abound. There must be dozens of them. In the simplest sense, a field guide is a smallish book with lots of photos and some description and range information. It exists to help you identify a bird in the field. It is not a natural history source. Natural history, ecology, and evolution are great subjects but are not appropriate for the field unless you have hired a porter or two to help you carry the books.
Some guides use actual photos of birds but these often lack detail and are somewhat limited by the fact that they show species in a specific part of its range, but do not generalize that appearance of that bird so that you can easily use the photo elsewhere.
Some guides actually take photos from a number of locations and digitally generalize them. This is very helpful.
Some guides use artist drawings of the birds and these are often excellent.
Good, but brief range and habitat descriptions can be as valuable as the images and ought to be featured for each species.
The birds ought to be ordered in the book in some logical fashion - generally books use a taxonomic system which makes a great deal of sense.