Spice and Wolf, Vol. 16: The Coin of the Sun II (light novel)

(Volume 14)
Having arranged to receive a map of the northlands from the silversmith, Lawrence's pleasure at imagining that he'll be able to travel to Yoitsu with Holo lasts but a moment, for when they revisit the town of Lenos, they immediately become entangled in the schemes that surround a forbidden book. The book is said to contain forbidden arts that might threaten Yoitsu itself, but to get it, Lawrence would have to return to his trading route, leaving insufficient time for him to go with Holo to Yoitsu. Faced with the harsh reality of this decision, what will Lawrence's solution be?!