The only thing knitters love more than yarn is knitting books. If you don't believe me, take a look around the places where they like to relax: A quick study of my own environment revealed knitting books in the following locations (I swear I am not making this up):
Stacks covering both ends of my desk
On, around, and under the living room sofa
Filling 4 full-height bookcases
Piled on both nightstands in my bedroom, in addition to the bookcase at the foot of the bed, and all over the floor around it.
Several rogue volumes arranged on the kitchen island, where I was (allegedly) stirring something, and reading at the same time.
Disclaimer: I didn't check any of the bathrooms, because neither of us want to know about that.
The point is, your knitter has a wishlist of books as long as both our arms; I guarantee it. If you're lucky, an official inventory has been created at some big online bookselling website, and all you have to do is ask. If not, refer to the envelope-of-cash technique explained under "Tools", above. Your knitter will know just what to do.
I hope these ideas will help you find the perfect surprise for the knitter(s) you love. Feel free to forward, link and post this list wherever you think it might help a soul in need. And please, leave a comment if you know of any I forgot!