If you search Ebay and use advanced search to see completed items you can check each book to see if any are worth making the effort to sell individually.

For the others the simplest option would be to do a job lot and get rid of them all in one go. Knitting books tend to be quite heavy so it may need to be collection only, unless your happy to package and possibly wait in for a courier.

I'd list under crafts - Knitting rather than books as your likely get a better price there.

A job lot won't get you as much as selling individually but a single transaction is a lot less hassle than 30.