Kroodsma's first book, The Singing Life of Birds, was an instant classic. Birdsong is an excellent follow-up, as it deals with the same subject, but in a different manner. It's not nearly as technical, and is much more relaxed, for lack of a better word. Kroodsma focuses on particular, individual birds, and really tries to get to know them through their vocalizations. As he says, this book is not about identifying birds, but identifying with them. What he is able to learn by really listening is fascinating, and makes a great read.
Full Review of Birdsong by the Seasons