Here is a list of books for perfect olympiad combinatorics preparation.
For general study:
(1) A Path to Combinatorics for Undergraduates
(2) Principles and Techniques in Combinatorics
(3) Problem-Solving Methods in Combinatorics: An Approach to Olympiad Problems
For practising problem-solving:
(1) 102 Combinatorial Problems
(2) Combinatorics: A Problem-Based Approach
(3) The IMO Compendium: A Collection of Problems Suggested for The International Mathematical Olympiads: 1959-2009 Second Edition
For Olympiad Graph theory:
Olympiad uses of graph theory is a bit different from formal graph theory taught in university courses. The best book for this is
(1) Graph Theory: In Mathematical Olympiad And Competitions
(2) IMO Training 2008: Graph Theory
For probabilistic methods in olympiad combinatorics:
(1) Expected uses of probability
(2) Unexpected uses of probability
For generating functions and recurrence relations:
Generatingfunctionology
For combinatorial inequality type problems:
Combinatorial Extremization
For various advanced techniques:
Extremal Combinatorics
For elementary combinatorial problems with geometric flavour:
Elementary Combinatorial Geometry
For ultimate problem solving (hard):
Problems from the Book
Pranav A. Sriram's book contains more than enough higher combinatorics contents which are only needed to tackle notoriously difficult (but not so elegant) Chinese TST problems. But what I have listed is enough for achieving success in EGMO or even in IMO!
Happy Problem Solving!