As you would expect, even when accounting for your hiking books, the size of the backpack you need will depend on the duration of your trips. Backpack capacities are measured in liters, and this is the reference point you need to use.
For just a few days or even a day, you can get away with packs from 35 to 50 liters. But, if you want to indulge in week-long hikes and want to bring some extra books, look for upward of 70 to 80-liter packs.