According to a rough estimate the western manuscripts collections and (private) archives contain some 500,000 letters, of which over 300,000 are catalogued. The letters kept in the University archives are not catalogued; the same holds for most letters kept in the oriental collections. They can be found through collection guides.
- Learned correspondence – Includes: Justus Lipsius (1547-1606; partly catalogued); Bonaventura Vulcanius (1538-1614), Carolus Clusius (1526-1609) (online edition); Christiaan Huygens (1629-1695) (online edition), André Rivet (1572-1651), Prosper Marchand (1678-1756), Willem Lodewijk de Vreese (1869-1938), Johan Huizinga (1872-1945), Kornelis Heiko Miskotte (1894-1976), Cornelis Hendrik van Schooneveld (1921-2003).
- Literary correspondence – The collection of the Maatschappij der Nederlandse Letterkunde contains many thousands of letters either written or received by famous Dutch authors. Examples of these literary correspondences include: editorial board of the famous journal De Gids (LTK 1888, LTK 2241), letters to and from Constantijn Huygens (1596-1687), Nicolaas Beets (1814-1903), A.C. Kruseman (1818-1894), Nico Rost (1896-1967). The archives of A.W. Sijthoff (Leiden) and De Erven F. Bohn (Haarlem) publishing houses contain correspondences with literary authors, especially dating from the second half of the nineteenth century.
- Other correspondence – This involves correspondence in institutional archives, such as the University archives, the archives of the Maatschappij der Nederlandse Letterkunde and business archives such as that of the publishers Sijthoff and Bohn.
- Collective items – Single letters are bought by the library on a regular basis, or received as a gift. These acquisitions have been (and still are) placed under collective shelfmarks; for the library: BPL 885 (until 11.1906), BPL 1886 (until 1.1.1985), BPL 3100 (since 1.1.1985); for the collection of the Maatschappij der Nederlandse Letterkunde: LTK 1004 (until 1.1.1985), LTK 2175 (since 1.1.1985).
The online catalogue contains:
- Descriptions of letters or correspondences. The online catalogue offers the possibility to limit your search to Letters. Then use the name of the sender and/or recipient as a search term. Correspondences: for example, the letters of Huygens to Leibniz and those of Leibniz to Huygens, under shelfmark HUG 45, are described in separate records (and also stored that way).
- Descriptions of complete letter collections and archives containing correspondence in collection guides, amongst others: Beets. — Bohn. — Bolland. — Burman. — Cunaeus. — Huizinga. — Huygens. — Kinsky. — Lipsius. — Luzac (fam.). — Maatschappij der Nederlandse Letterkunde. — Marchand. — Miskotte. — Van Papenbroeck. — Sijthoff.
Digital Collections is a database containing digital images of manuscripts, archives, letters and other materials (such as rare books, maps, prints, drawings and photographs). More and more letters are digitally available. For example, the correspondence in the archives of De Gids, Tideman and De Vreese, just as the correspondence in the archives of Beets, Kruseman en the publishers Sijthoff en Bohn. In time the letters in the archives of Johan Huizinga and Christiaan Huygens will also be digitally available.
The older letter collections are described in two printed catalogues (completely incorporated in the online catalogue):
- Geel 1852 — Describes the correspondences acquired in the years 1716-1852. The senders have been processed in the authors index.
- Catalogus compendiarius 1932-1937 — Printed index to the correspondences of all letter collections (and alba amicorum). Pars II.1 (= dl. 22), II.2 (= vol. 26) give alphabetical lists of names of senders and receivers for the library collections, with reference to the shelfmarks; pars III (= vol. 34) does the same for two loan collections: Maatschappij der Nederlandse Letterkunde en Remonstrants Seminarie.