The Library contains approximately 135,000 printed volumes. About half are pre-1851, including incunabula, and approximately 20,000 are in the English Short Title Catalogue (about half of which are pamphlets), covering Christian theology, ecclesiastical history, other religions and philosophy, most subjects in the humanities and many scientific books. Dr Williams's Library is the main research library of English Protestant dissent, in addition to the printed collections there are around 90 major manuscript collections.
Traditionally the collection has concentrated on three subject areas, namely Byzantium, Biblical materials and theology, as well as church history. The stress in recent years has moved noticeably onto the history of Protestant dissent which reflects the manuscript holdings. The collection is rich in autobiographical material and seventeenth and eighteenth-century correspondence. It also contains a small but interesting number of portraits of dissenters, photographs, prints, maps, and various other objects and artefacts.
Specialist printed collections held in the Library include the Walton Collection (probably the best collection of English Protestant mystical material available), the Norman Baynes Library of Byzantine history and culture, the Liberation Society Library, and the books and manuscripts from New College, London. The latter includes the libraries of some of the most important early dissenting academies, such as that of Philip Doddridge at Northampton.
There are a number of important collections for English literature. The George Henry Lewes Library, and the Henry Crabb Robinson manuscript collection are particularly noteworthy. Robinson corresponded with many leading German and British Romantic figures of the first half of the nineteenth century, including Coleridge, Charles Lamb, Wordsworth, Southey, Walter Scott, as well as the anti-slave campaigner, Thomas Clarkson, Elizabeth Reid (who founded Bedford College for women) and Harriet Martineau.
Items in the manuscript collections date from the sixteenth-century with the rare survival of a puritan minister's diary 1587-1590 and the 'Second part of a Register'. The strength of the archive lies in the seventeenth-century, with the papers of Richard Baxter, John Quick (Lives of French Protestant ministers) and Roger Morrice, whose ‘Ent’ring Book’ is a key resource for late seventeenth-century history. The Library also holds the original minutes of the Westminster Assembly which were published in 2012.
The eighteenth-century archive also offers a rich source of material and includes the correspondence of Philip Doddridge, Theophilus Lindsey and the theologian and scientist Joseph Priestley amongst others. The nineteenth century that of James Martineau and Thomas Belsham. Many of the records of the major nonconformist institutions of the nineteenth and twentieth century have been deposited in the Library, such as the British & Foreign Unitarian Association, The Inquirer Publishing Company, the General Assembly of Unitarian & Free Christian Churches, and the Free Church Federal Council. Other collections on deposit include the Presbyterian Fund, the Congregational Fund and the records of the General Baptist Assembly.
The Library also holds many primary sources relating to the history of nonconformity from the sixteenth to the nineteenth-centuries.