Books:
Putnam And Beyond, T. Andreescu
The IMO Compedium, D. Djukic and V. Jankovic and I. Matic and N. Petrovic
The Art and Craft of Problem Solving, P. Zeitz
Problem Solving for Math Competitions, Harm Derksen
a prime for mathematics competitions, A. Zawaira and G. Hitchcock
IMO Shortlist 2016
IMO Shortlist 2020
Past Problems: Pavlos Tzermias's Putnam list poroblems
Lecture notes: