The hardest book I ever read while running was Crime & Punishment.
The most grueling was John Adams.
The most engrossing was Angels & Demons.
The ones that pushed me were Murakami’s.
By splitting focus into 1. Mental 2. Physical, I force myself to do both well, spreading then reintegrating my attention on a broader level. That’s probably a good clue regarding how I live on two levels simultaneously.