This manual of leather working by an unnamed author describes tanning, skinning, and preserving hides of underdark animals. It includes knowledge of leather armor and similar goods. One learns secrets of leather magical goods, some taxidermy, and how to id hides. 

Upon finishing the complete work totaling 800 hours or 100 long rests, and making an Int or Dex check DC 13 at halfway point and 16 at the end, the reader gains proficiency in Leatherworker’s Tools. 

Notes: Proficiency: Leatherworker's Tools