A Mathematical Olympiad Primer is an excellent guide for young mathematicians preparing for competitions, such as the British Mathematical Olympiad. The book contains theory including algebra, combinatorics and geometry, and BMO1 Problems and Solutions from 1996 onwards. This is the revised and expanded 2nd Edition.
Perhaps not the greatest overview of theory, but that's not what you buy it for. The useful bit is the set of solutions to all BMO1 questions of recent years, which aren't available online.