I've now been vaguely into bird-watching for a couple of years, and have a few books on the subject. Except most of these books are basically pictures of birds, plus maybe some advice on where to find them. Nothing that's really "how birds work."
So, finally I bought this book, and it fills the spot nicely. It truly is just the essentials - it's an academic text on ornithology, but at 150 pages it's very much just an introduction to the subject. There are many references to landmark papers for those who are interested (hint: I'm not that interested!) and side bars on specific pieces of research, but otherwise it's pretty straightforward.
My previous forays into areas I don't know about has been to grab a giant authoritative textbook and work my way through it (like this). This book was something different, being a small, light and unassuming text. While designed to be some part of a degree course, it felt kind of like a textbook for an A-level in birds!
I think it's somewhat pricey for its size if bought new (I bought second-hand), but otherwise I'd highly recommend it as a quick introduction to ornithology.