Birding is, as much as anything, a hobby of details. As you progress in the hobby, you need to spend time learning the fine details of various birds. Most birders start out with a field guide, one that they carry with them as they bird. Rarely does it stop there, it seems.
When I’m birding, I’ve mostly switched to electronic field guides (which are a posting for a different time), but I still carry one of the standards, the Sibley Western Field Guide. At home, I have a copy of the larger National Sibley. I also have a copy of the National Geographic Field Guild. They are good examples of the two schools of guides, with Sibley being painting-based and the National Geographic being photo-based.
It may seem like a photo-based guide would be the best, but I’ve found in practice the Sibley, based on drawings, works better for me. The big philosophical difference is that National Geographic images all show a specific bird at a specific time of year, while the Sibley images are idealized images of the species, with a focus on the marks you use in the field to help you understand which identifiers are most important. Under most circumstances, the drawings help you more than photos, but there are times when only photos answer a question. The quality of a photo-based guide depends a lot on both the quality of the images, but also how well the editors choose representative images of a species.
Sometimes, however, what you need is lots of images. Nothing defines the complexity of birding more than gulls, which a few birders absolutely love for the challenges, and many birders grumble about at the same level as mopping the kitchen floor. It’s an occasionally dirty job, but you gotta do it, at least once in a while. But if you do, you’ll quickly find most general guides can’t cover the complexity. Gulls change their plumage as they mature in major ways over the first few years, and especially with younger birds, the differences between species can be subtle and individual birds vary widely from the standard. In my view, the birders who can pick a Slaty-Backed third cycle gull out of a flock of 5,000 mixed Herring and Western gulls gets nothing but respect from me (and I know damn well even if I had the patience to sort out that flock, I’d still never see that bird).
But when you start playing with gull ID (or shorebirds, another class of birds that can make you crazy), you need specialized guides with a lot more detail.And that’s why I own Gulls of the Americas and The Shorebird Guide, because somedays, you need to be able to sit down with your images and be able to make heads or tails of a 3rd Cycle Glaucous-Winged or understand the difference between a Least and a Semi-Palmated Sandpiper.
So those four books are my go-to library where I research about 95% of my birding questions, supplmented by my electronic guides I carry in the field, and resources like Flickr or Cornell’s All About Birds.
But most birders build a library of books over time, because when the weather doesn’t cooperate, you can still sit down and read up on the hobby.
If you’re just growing past the “carry around binoculars” stage and don’t really know what that means, the Natgeo Birding Essentials guide is a good starting point. Owls has been a recent fancy of mine, and so I’ve gotten a couple of guides to stary studying them. I particularly like the Field Guild to Owls of California and how it describes and discusses the birds.
Finally, you can’t find birds if you don’t know where to look, so every birder ends up grabbing a stack of these regional guides. My favorites here in the bay area are Birds of San Francisco and the Bay, John Kemper’s Birding Northern California, and for those of us here in Santa Clara County, Birding at the Bottom of the Bay, which is available through Santa Clara Valley Audubon. The more general guides are good ones to get you started and help you explore the highlights around a region, but the more you want to explore, the more you’ll find yourself drawn towards the specialty guides done by the local Audubon chapters. Many of these are now going online, and a great example of what’s possible is done by Sequoia Audubon in their San mateo County birding guide — this really is the future of birding guides, I think.