I have a copy of the Sibley's and the Nat Geo's Birds of America. I prefer the Nat Geo for the images. My copy of the Sibley's has a cover on it that looks to be water resistant.
You might also be interested in iBird if you have a tablet computer to display it on. You can't thumb through the pages, but if you already know what bird it is, it has much more information than the field guides.