Books appear in several contexts:

In-Story Books
The expedition team has a hidden agenda of finding original pre-Rash books in the Silent World which can then be sold to (mainly Danish) interested parties. Books are very expensive, as they have to be (re)produced by manual labor and the modus operandi of the cleansers isn't conducive to obtaining more old ones. (Some books, most prominently Allt om Spadar, seem to see a lot of profaning use, though.) However, books are nonetheless a primary means of recreation.
Books of the Comics
aRTD has had a (single) print run, and the first book of SSSS exists in two editions. Volume 2, as well as another reprinting of volume 1, are in the making after successful Kickstarter campaign.
Books as a Source of Inspiration
aRTD drew heavily on the concepts of the Kalevala. No sources of inspiration beyond a couple horror movies have been named for SSSS yet.