I didn't see a thread on it in the last dozen pages, so I thought I'd throw out a few ideas for the use of mundane books.
Elements to be introduced:
Allow the processing of raw skins into vellum at the tanner's workshop, and of rope reeds into papyrus at the farmer's workshop.
Introduce bookshelves, and allow for rooms to be designated as libraries/studies.
Writing:
Using vellum/papyrus, a unit of leather for binding, and a unit of dye, allow dwarves with a certain skill (perhaps merge it into reading and call it Literacy?) to write books, with the labor itself being requested from a library room.
Allow dwarves to write two types of books: casual and academic.
Casual books may be read for a very positive thought. Dwarves with personalities which lean towards an enjoyment of reading will do this automatically during their breaks. The strength of the thought is relevant to the quality of the book. The books may be fictional or historical.
Academic books may be written by dwarves with at least a certain rank in a given skill (other than Literacy, of course). A dwarf may write these books to impart their knowledge into a form which others may Study. Dwarves with a certain labor designated and no task for some time may be assigned to study their profession, though like military dwarves they will gain negative thoughts from being forced to study for too long (except, perhaps, if they fall in the same category of dwarves which enjoy reading.) The speed at which they learn from a book would depend on the student's learner skill, the quality of the book, and the student's literacy skill; the level to which they may learn is capped at one level below the author's skill level at the time of writing. When writing a professional book, the quality will either be affected by the author's teacher skill rather than their literacy skill, or (if the code allows) be a function of both. The speed at which the book is written will be directly affected by the author's literacy skill.