Identification guides are an important component of your library if you plan to have anything to do with nature. From hiking to gardening to living in the country, identifying what you see can make the difference between just seeing nature and truly understanding it.
I have read literally dozens of bird identifications books since I was a kid. I have often liked one or two aspects of them, but none of them have been anywhere close to the level of The Sibley Guide to Birds. The big issue with most bird identification guides is that while they may show a photo, it is usually a single photo, of one bird, of questionable quality, in one season, in unique lighting. If you happen to see a bird in that same time of year in the same conditions, then you have a reasonably good mode of identification.
Sample pages from The Sibley Guide to Birds.
Left: Golden Eagle; Right: Bald Eagle
The Sibley Guide is all illustrated with paintings done by David Sibley. Many of the birds have multiple illustrations showing immature, mature, regional, and other variations. As you can see from the pages above, the Golden Eagle has twelve illustrations for identification... 12! That is amazing! While each individual illustration is good, they may not be the absolute best artistically - meaning they may not look as good as some amazing painters recreate life. But what Sibley excels at is capturing the unique characteristics of each bird. It's almost like how a caricature artist captures the two to three unique features of a face and portrays the image so anyone can identify that person. Sibley's paintings are more realistic than that of a caricature artist, but he is so talented at capturing the unique points to identify each species of bird.
Overall, this is the best bird identification book I have ever seen and used... and I have used it often. I highly recommend this book be added to your bookshelf.