There are some books available for olimpiad type preparation.
Locally I know of a book produced by Suhaimi one of the ex-IMO contestants and the current IMO team leader. Read more here:
There are also some books / resource about past OMK questions and solutions, but I don't know much about them so you might have to dig it out by yourself.
As for foreign book, this book is my all-time favourite:
Yes, expensive, but very enlightening and helpful. Even if you are not preparing for Maths Olympiad, this book is still very empowering and eye-opening.