Books for combinatorial thinking
Solution 1:
You might want to check these out (there are a coupe of others, but I am not home and the titles are escaping me).
Proofs that Really Count: The Art of Combinatorial Proof (Dolciani Mathematical Expositions)
Principles and Techniques in Combinatori, Chen Chuan-Chong, Koh Khee-Meng
Applied Combinatorics, Alan Tucker
You might want to look at Donald E. Knuth - The Art of Computer Programming - Volume 4, Combinatorial Algorithms - Volume 4A, Combinatorial Algorithms: Part 1
I'd also recommend books on problem solving, for example:
102 Combinatorial Problems, Titu Andreescu, Zuming Feng
Combinatorial Problems in Mathematical Competitions (Mathematical Olympiad), Yao Zhang
Combinatorial Problems and Exercises, Laszlo Lovasz
Regards
Solution 2:
Concrete Mathematics has a feast of lip-smacking mathematics in it.
Solution 3:
I'd take a look at Proofs that really count
Solution 4:
H.J.Ryser, Combinatorial Mathematics
M.Hall, Combinatorial Theory
Solution 5:
I am working on James A Anderson's book called Discrete Mathematics with combinatorics and I'm pretty happy with it. I like the combinatorial part of mathematics the most and this book does try to look at things from that point of view on many cases. Although you should really pick it up at a library and look for yourself.
It is not however very problem oriented. You can try concrete mathematics by Knuth, Graham and Patashnik. Also, I find the problems on Van Lint and R.M WIlson's book really good (those are just in combinatorics though).