A Friends group can help raise funds for special library projects. Friends groups are excellent at attracting publicity and encouraging good public relations and good will for a library. A foundation, described above, may act as a Friends group.
The Association of Library Trustees, Advocates, Friends and Foundations (ALTAFF) website is a good source of information, particularly their Fact Sheets, http://www.ala.org/united/friends/factsheets. Another resource is Friends of Libraries Sourcebook, published by the American Library Association and available to borrow from Iowa Library Services.