The Library of the Western Reserve Historical Society is the largest American history research center in northern Ohio. Its holdings consist of 238,000 books; 25,000 volumes of newspapers; 30,500 rolls of microfilm, 4,000,000 prints and photographs; and more than 3,700 collections of manuscripts and archives that comprise more than twenty million items. The Library is the principal repository for histories, records, and papers relating to the growth and development of Cleveland and that portion of northeastern Ohio once known as the Connecticut Western Reserve.

One of the Library's principal strengths is its manuscript collections that touch upon every aspect of Ohio's history. These collections cover a wide range of subjects including architecture, education, charity and social welfare, farming, immigration, law, politics, and transportation. Among the individuals represented here are abolitionists, ministers, lawyers and jurists, industrialists, journalists, and political leaders, including three Ohio Governors, six United States Senators, seven United States Representatives and two Ohio-born Presidents: James A. Garfield and William McKinley. The photographic holdings are vast and cover all subjects dating from the advent of photography in 1839 to the present.

The Society was one of the first institutions in America to collect genealogical materials and is, today, one of the largest family history research centers in the country. Although the focus of the collections is on source materials for the region east of the Mississippi River, major sources for other states are collected, especially through donation.