[copied from the British Library landing page]
The Incunabula Short Title Catalogue is the international database of 15th-century European printing created by the British Library with contributions from institutions worldwide.
The database records nearly every item printed from movable type before 1501, but not material printed entirely from woodblocks or engraved plates. 30,375 editions are listed as of March 2014, including some 16th-century items previously assigned incorrectly to the 15th century. Additions and amendments to ISTC are made frequently, and new information and comments and suggestions are always welcome by e-mail at email@example.com.
Information on each item includes authors, short titles, the language of the text, printer, place and date of printing, and format. Locations for copies have been confirmed libraries all over the world. Many links are provided to online digital facsimiles, and also to major online catalogues of incunabula such as the Gesamtkatalog der Wiegendrucke http://www.gesamtkatalogderwiegendrucke.de/GWEN.xhtml and the Bayerische Staatsbibliothek Inkunabelkatalog http://inkunabeln.digitale-sammlungen.de.