LAS VEGAS—Along with packs of ice, Gatorade bottles and the occasional gym bag, it’s not uncommon for NBA players to be seen toting personal items out of practice sessions. Still, it was little unusual to see Kristaps Porzingis, the Knicks’ 20-year-old Latvian rookie, clutching a bright orange book under his arm as he exited the gym after a summer-league practice a few weeks ago.

“Coach [Derek] Fisher gave it to me. It’s called ‘The Positive Dog.’ Took me maybe three days to finish, and was simple to understand—explained in...