Putnam and Beyond takes the reader on a journey through the world of college mathematics, focusing on some of the most important concepts and results in the theories of polynomials, linear algebra, real analysis in one and several variables, differential equations, coordinate geometry, trigonometry, elementary number theory, combinatorics, and probability. Using the W.L. Putnam Mathematical Competition for undergraduates as an inspiring symbol to build an appropriate math background for graduate studies in pure or applied mathematics, the reader is eased into transitioning from problem-solving at the high school level to the university and beyond, that is, to mathematical research.
Obviously a go to for Putnam prep, but beware that it gets very difficult quickly. The first chapter all the problems are gold, but as you go it becomes very abstract. The problems start to be best for A4/B4 and up. For I certainly first recommend "The art and craft of problem solving" by Paul Zietz for performing decently (top 500) on Putnam.