where do i build books?

You can't, they are obtained by trade and can randomly be made by workers.
Please read the mod description for more info :)

NEW RESOURCE: Books
Research is done with base knowledge, coming in the form of BOOKS
With some books on hand, academics no longer need to experiment blindly, wasting tons of vakuable ressorces.
* Sources:
- Can be purchased at the castle market or other trade building (mod: trade dock)
- Occasionally, an educated craftsperson will compile the experience from decades in the craft into a book.

Books need to be more frequent. My village has been running several years and I have over 150 citizens and a keep but still no books. The only research left requires books, which means most of the content is still locked out due to the inability to research it.

Have you tried buying some? They are not expensive:)
Initially books very only meant to be obtainable by trade, the random books made by craftsmen are a bonus.
As with everything including chance, there will sometimes be more and sometimes less. I've also been told books are made too often

Well I have 31 years in game and about 180 abitants. Got automatically 3 books. Have not activated the market yet (still have to learn how and not trained donkey yet).

I wonder if the Tanner and leather production is related in any way. I have a tanner and a lot of leather.