Nearly every recurring library book sale in the United States is organized by a Friends group rather than by library staff. Proceeds typically fund things the operating budget does not cover: summer reading prizes, museum passes, author visits, new furniture, or technology.
Understanding who runs the sale helps you shop it. Friends groups set the pricing, the preview night rules, the bag day schedule, and the donation policy. Many also operate a year-round shelf or storefront inside the library, which is often the best place to find inventory between big sales.
Membership usually costs $10 to $25 a year and frequently includes early access to sales. If you shop the same sale more than twice a year, membership tends to pay for itself in first-pick access alone.