There are now a large number of books and workshop manuals available from the Citroen DS.

Workshop Manuals
Thanks to the huge efforts of the Citroen owners community, most factory manuals are now available in PDF format. You can find the list of available workshop manuals here: https://sites.google.com/site/citroenpublicationslist/

Books

There are a number of commercial books available on the DS, the best I own are:

Original Citroen DS by John Reynolds

This is finally being reprinted and contains some of the best reference photos of the DS for restorers. It also has a lot of production data including production numbers, colours, fabrics etc.

Citroen DS: The World's Most Beautiful Car

As great as the John Reynolds book is, I find this book is the perfect companion, showing many full colour photos which are really helpful when figuring out how the car should be put together.

Goddess by Charles Vyse

Chas Vyse was very active on the DSeries Yahoo Groups and went on to product this excellent book detailing the restoration and maintenance of his DS.