These are the kinds of books you get if you are either a scientists studying bird migration and related issues, or a very serious bird geek. The first two can be obtained at very low prices used, but the third will set you back at least 50 bucks US$ if you want a used copy. Note the spread of publication dates. It is not the case that the oldest book is out of date in all respects: Quite the contrary. Alerstam reviews theory and ideas that have not been revisited or revised to any great degree. Also, it is interesting to see how changes in the field develop over a decade or so. In any event, I've labeled the books by year of publication to make it very clear that I'm not showing you hot off the press items here.
1993: Bird Migration by Thomas Alerstam is a general overview of bird migration with an excellent overarching discussion of the context in which migration has evolved followed by a focused study of nine different ecologically defined categories of birds. There is also a detailed discussion of what was known about navigation at the time of publication.
2001: Bird Migration: A General Survey by Peter Berthold covers similar topics as Alerstam but with more focus on the evolution of migration, methods of study, bird physiology and threats to migratory species.
2005: Birds of Two Worlds: The Ecology and Evolution of Migration, Edited by Russell Greenberg and Peter P. Marra is different from the above mentioned book in that it is an anthology of scholarly papers on bird migration, covering the full range of migratory syndromes and the evolution of migration.