The book should be easy to read, or at least average. The level for reading such a book is the MATH and CS school curriculum. It should also be expanded on topics related to CS, and also be narrated in a light manner. It should be kind of in a intro tutorial before starting a middle combinatorics.
P.S need such combinatorics after which I can understand the course of data structures and algorithms. Also understand the course of computing algorithms.
Combinatorics is important if you wish to be a competitive programmer.Go through concepts from the given pdf and try to practice as much as you can from platforms like codeforces and codechef.Combinatorics PDF