[scorecard]

Art of Problem Solving is popular with the Math Olympiad types. I see that others on this thread have recommended it already.

https://artofproblemsolving.com/

[crawftv]

I second this. Do t be scared off by it being for math Olympiads. A lot the first volume deals with concepts across much of the field. Lots of practice and ideas for logs, exponents, word problems. And it comes with a solution guide which helped me a lot.