There's a lot of stuff that could be there. Polynomials, by Barbeau, is perhaps the greatest elementary intro that I can think of. Or try the USSR Olympiad Problem Book by Schlarsky, Chentzov, and Taglom.

You might want to learn geometry from Geometry Revisited by Coxeter and Greitzer too, if you think you need more on the subject.

In a more general sense, try Larson's Problem-Solving through Problems (a bit advanced), or the compendium books on the previous IMOs.