The worst feature of the book is the fact that only one (unintuitive) model for the theory is provided. Discussion of the significance of the results obtained is not particularly useful - probably anyone smart enough to solve the puzzles will not find anything there that they couldn't figure out for themselves.
But nevermind: if you want a good introductory course in combinatory logic (or you want to understand (a version of) Godel's 1st incompleteness theorem), then I would recommend this book for you!
The birds are functors that compute on strings.Self reference comes into play when the Mockingbirdshows you what a fixpoint computation is.
The phethora of birds may confuse you if you try to read itfast or skip solving the puzzles. The problems are noteasy, it took the mighty mathematical titans - Turing and Godelto provide the initial solutions. If you are stuck,Smullyan provides all the solutions at the chapter end. It requires concentration and remembering previous tricks, something akin to solving Rubik's cube without a solution guide.You will love it if you love chess problems.
In the end you will come out with a deep sense of accomplishment having understood the proof of Godel's incompleteness theorems, Combinatorial Logic, and Functionalprogramming, when all you thought you were doing was figuringout puzzles of birds.
Hard to find book, but its worth its weight in gold. The other book to complement this is "Forever Undecided,a puzzle guide to Godel" by Smullyan, it uses Modal logic puzzles to motivate you, but the end result is the same.
Remember Smullyan is Professor of mathematics and logic,he is classy and witty like Knuth. Don't confuse himwith the popular mathematical journalists.
- Mosh http://www.cs.albany.edu/~mosh