Anyone with a programming background getting into R should absolutely go read _Advanced R_. I've been using R off and on for a while now but Advanced R was a real revelation. All of R's weird behavior finally made sense.

Edit: Also there is a 2nd edition in the works: (confusingly hosted at the same subdomain of a different version of Hadley Wickham's website. https://adv-r.hadley.nz/