The collections of the Mercer County Library System can be used free of charge by anyone who lives, works, or owns property in East Windsor, Ewing, Hopewell, Lawrence, Robbinsville and West Windsor Townships and Hightstown Borough. Since the Mercer County Library System cannot acquire and retain all available materials, it must utilize a policy of selective acquisition and collection maintenance. Collection development is a judgmental, interpretive set of processes applied by professional librarians. These processes require: knowledge of community needs; familiarity with the library system's collections and appropriate bibliographic tools; a general knowledge of subject literature; and consideration of the library's financial resources.

The library's collections include materials in a variety of formats, at many intellectual levels, to serve a wide range of ages, reading, listening and viewing skills, and educational backgrounds. Its collections are developed to provide the resources needed by its service population, in accordance with the Library Mission Roles and Goals Statement. Staff members assigned to collection development for any library branch shall obtain an accurate profile of the population served, in order to recognize and meet its needs. Public demands shall be treated as an indicator of need. Because resources should reflect the changing requirements of our patrons, collection development policy shall be subject to continuous review.