Well, a book that taught me lots of nifty concepts back in highschool was "Problem Solving Strategies" by Arthur Engel. You'll find it to be a very resourceful book throughout highschool, and maybe even into university. However, be prepared to do some work on your own too, as this book depends on how much work the reader is willing to put into it. I can try looking for an ebook for you (I highly recommend this book, as I believe it is a must have for highschool students preparing for Math Olympiads).