Don't have the update yet, but read this online earlier:
Chapters for this motif (or rarely, the full book) can be acquired in the Unidentified Equipment boxes obtained as a reward for performing Heists to the Thieves Guild’s highest standards.
You have to do the heist daily quest in the Thieves Guild DLC. They can only be done once a day. The motif also only drops from the "Unidentified Equipment" bag you get for the completion. So far I have received a motif page every time. HOWEVER... the bag only drops when you complete the heist perfectly. That is doing all objectives and within the time frame that is set. Try not to be seen and avoid fighting as much as possible if at all. Good Luck.