So far we know that books need leather to be crafted, lether is in other words skin so why not being able to also use warg fur, lion fur etc. for the same reason since they are too skins from different animals. Also not all are able to secure livestock such as Arowk (sry can't remember name). People may live near areas that spawn many wargs, lions etc. so that would help them have a source.
Covers: After the normal book is ready players could also add 1 piece of warg fur, lion fur etc. to change the books cover and also be able to change it's color.
P.S ik books are rather useless now since no more enchant but still usefull just for bookselves and decoration :P