The best combinatorics book I’ve read so far is ‘Combinatorial Extremization’, by Yuefeng Feng.
The book is divided into 13 chapters. Each of them covers a particular combinatorial technique you can use to solve your problems. Some of them are easy to understand, but in general, the level of difficulty of this book is quite high. The main focus of each chapter is its exercises: in combinatorics, you need very few theory and a lot of intuition and practice. Also, there are some very nice end-of-chapter exercises with solutions at the end of the book. The requirements to read that book aren’t actually a joke: inequalities (in chapter 1 already they use the Cauchy-Schwarz inequality), trigonometry, convex/concave functions.
My favorite chapter is the eleventh (page 111): ‘Counting In Two Ways’. It talks about double counting, which is a technique in which you create an equality or an inequality by counting the same quantity in different ways. The main reason why I appreciate this technique is because everyone can understand it without having to know anything else.
I also wrote in my blog about some exercises solved with double counting: (https://www.mattiagiuri.com/2020/11/17/double-counting-part-1/)
To sum up, I really recommend this book to people who are already familiar with some Mathematical Olympiads topics and would like to improve their skills in Combinatorics.