Book recommendation : Olympiad Combinatorics book Can anyone recommend me an olympiad style combinatorics book which is suitable for a high schooler ? I know only some basics like Pigeon hole principle and stars and bars .
I hope to find a  book which contains problems which purely test our originality ( the problems with beautiful constructions like USAMO 2017 -TSTST P2: Which words can Ana pick?, Nim problems, games,tillings, etc ) . More specifically problems which doesn't require theory but requires out of the box thinking .
I don't know much about recurrence relations, generating functions or graph theory, so I would also love to see a book which introduces these topics .

One possibility is Problem-Solving Methods in Combinatorics: An Approach to Olympiad Problems by Pablo Soberon.  As the title says, it's intended to prepare the student for Olympiad problems, and the author won a gold medal in the International Mathematical Olympiad.  Some of the exercises in the book are drawn from recent Olympiads.
Coverage includes the pigeonhole principle, graph theory, generating functions, and partitions.
