Ask your librarian about existing book clubs in the area. He can probably tell you if the area is saturated with book clubs or if other individuals have inquired about joining one. Your librarian himself might be interested in joining your club.
Advertise by posting signs around the library. Some libraries have a monthly newsletter, and if your club has the backing of the library, you can probably list a call for members. Don't forget to try listing your call for members in your local newspaper as well.
Get involved with other clubs or organizations that meet at your library. If you begin to widen your circle of acquaintances and friends you might be able to recruit some new members.
Inquire about using library facilities for your book club meetings. Book clubs often succeed when members can gather at a central location, and there's little pressure on individuals if they don't need to host the club meeting at their home.
Request that your library make books available for your club. Libraries often get discounted rates on books, and they can sometimes pass those discounted rates on to book club members. By ordering a limited amount of copies each month the library can keep from losing money while still offering a service too book club members on a first come, first served basis.