You will get this naturally on your way to winning the campaign. These books are either in a specific location (so watch out before making any form of alliance with the settlement holder) or by a rogue army. Those will be most likely a bit too tough for you at the start of your campaign, but can be beaten easily later on. They tend to wander as well, so be sure to have access to the area in which they walk around, lest you start unnecessary wars.