When I first got into birding while living in Arizona, I cruised on down to the Superstition Springs Center mall and browsed through all the bird field guides at Borders. I knew nothing of field guides, how to use them, or how to compare them. The Sibley Field Guide to Birds of Western North America just struck me as a good one. I liked the paintings, the descriptions, the maps, and the layout of the pages. It looked very intuitive and the price was reasonable, so I bought it!
Since that day several years ago I have lost three of them and purchased a couple as gifts for other beginning birders. It is still my go-to field guide. No field guide can even begin to display all the possible variations of birds as they may appear at sundry stages, but so far, Sibley's has been there for me to identify every bird that I have come across. I just bought my 4th copy. I may just have to permanently chain it to my body so I don't lose it this time.
Price: as low as $12.20 on several book selling websites