I like the American Bird Conservancy's Field Guide "All The Birds of North America". It's very compact and is designed to be carried into the field (plastic cover, rounded page corners, etc). Also the index contain a check-list, which is fun.
I've been using the American Bird Conservancy's All The Birds Of North America for about 20 years, and love it.
I also have an updated copy of Birds of Nova Scotia which was written in the early 1960s by the legendary Robie Tufts [1884-1982]. It's a rather thick book and therefore not a great guide to actually take into the field, but the text is fascinating and very informative.