Over 20 years ago, Kenn Kaufman's Advanced Birding was first published. It covered in detail many identification puzzles for North American birders. It was still useful when I became a birder many years later, and is still so today. But now there's a new edition - Kaufman Field Guide to Advanced Birding.
The new one is more of a supplemental volume than an update. Kaufman has chosen to focus more on building a framework for identification this time around, as opposed to species-level identification. Thus, there are more general principles and family overviews. But 10 chapters have been carried over from the first book that detail some problematic species. The chapter on empids alone is worth the price of the book.
Intermediate level birders will probably get the most out of this book. But anyone, even experts, can learn something from Kaufman. More details can be found in my full review:
Review of Kaufman Field Guide to Advanced Birding