Algebra for College Student
Algorithm
Approximation
Improvement

Books

Geometry For Colldge Students (Best Book)
Euclid and Beyond by Hartshorn
Geometry Revisited
Introduction to Geometry
Introduction to the Geometry of Complex Numbers
Problem Solving strategies for Math Olympiads
Solving Mathematical Problems by Terrance Tao
IMO Competition
Problem Solving by Halmos

More books for USAMO... Available here