The following is a non-comprehensive list of (potentially) useful resources that can be used to prepare to solve Putnam-style problems.
- Putnam training. At Northwestern University.
- Kiran Kedlaya's Putnam Archive. With problems, solutions, and results.
- Art of Problem Solving Contests Collections. Huge repository with enough problems to keep you busy forever. This site has a very active online community.
The electronic versions of these books are available courtesy of OU's library subscriptions as long as you are on campus. If you are affiliated with OU but away from campus, you can still access them by logging in to your account at the library's website and searching for the appropriate book.
- Putnam and Beyond, by Răzvan Gelca and Titu Andreescu. A problem-solving book aimed squarely at undergraduate students working towards tackling the Putnam exam.
- Problem-Solving Strategies, by Arthur Engel. An absolute classic in problem-solving.
- The IMO Compendium: A Collection of Problems Suggested for The International Mathematical Olympiads: 1959-2009, by Dušan Djukić, Vladimir Janković, Ivan Matić and Nikola Petrović.
- 104 Number Theory Problems From the Training of the USA IMO Team, by Titu Andreescu, Dorin Andrica and Zuming Feng.
- 103 Trigonometry Problems From the Training of the USA IMO Team, by Titu Andreescu and Zuming Feng.