I usually enjoy reading something thick and technical during my summer vacation, to learn something new and not necessarily work-related. This year, however, I am not sure which book to pick up. I would love to see what's on your summer reading lists.
My recent favourites:
Bayesian Data Analysis, 3rd ed, by Gelman et al
Designing Data-Intensive Applications, by Martin Kleppmann
The Art of Electronics, by Horowitz & Hill