I've got all the books mentioned thus far, and by far my favorite is
The Shorebird Guide by O'Brien, Crossley, and Karlson. I don't regret getting any of the others, but if I could only have one, it would be The Shorebird Guide.
A little while back Rick Wright reviewed it on his site -
www.birdaz.com/blog. Fantastic review, and described very well how I felt about the book as well. On his site, clike the "Book Reviews" link, and you should be able to find it.