Combined extreme value. Argument and Structure Feng Yuefeng, East China Normal University Press. This book is devoted to combinatorial extremum, with many novel ideas and methods, which is worth reading.
Graph Theory by Xiong Bin, East China Normal University Press. This book is one of the best in graph theory, but it is not necessary to read it all, just read 3/4.
Teacher Liu Peijie compiled a bound book, which is in the lecture series of proposer in mathematics competition. This book is just so-so, but it has many good topics and can be used as a problem book.
If your level is relatively high, I recommend Professor Shan Zun from Shanghai Science and Technology Publishing House. This book is a combination of basic topics and difficult rare topics. At the suggestion of Professor Leng, I made this book twice and gained a lot.
If you can finish reading these books I said, you will gain a lot. I wish you success in the game!