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Application Program Interfaces (APIs) are structured methods for one software application to communicate with another. APIs allow programs to interoperate and share data and services in a standard way. Here is a list of library-related APIs that library developers may find useful. If you have ideas for others that would be appropriate for this list, please contact me.

Only APIs that seem to be generally useful are listed here. However, almost any library catalog will have a search API (e.g., Z39.50) and may have others as well depending on the vendor and product. You can also find other APIs by using the directories noted below.

General Services

• Alibris - "Alibris is the world's largest independently owned and operated book, music and movie marketplace."
• Amazon
• Book Cover Images (LibraryThing)
• Chronicling America API (Library of Congress) - Historic newspapers
• Classify (OCLC) - Find Dewey and Library of Congress classification numbers for library items (e.g., books, DVDs, etc.).
• COPAC Search (SRU) and MODS XML fetch by identifer (major University and National Libraries in the UK and Ireland, including the British Library)
• Culture Grid search API - Search 730,000 object records from over 50 collections of national (UK), regional and local scope, plus collection and institution records. Culture Grid is a collaborative project of the Collections Trust, the Museums, Libraries and Archives Council (MLA), the European Commission and technical partners Knowledge Integration Ltd.
• DBPedia - Structured information from Wikipedia
• DOI Resolution (CrossRef) - Requires an account
• Google Book Search API - link to items in GBS, and find out about their availability
• Guess OPAC System (OCLC) - An experimental service that tries to intelligently detect the OPAC vendor and its ISBN,ISSN, and OCLCNUM linking template.
• Hathi Trust - Library-hosted books digitized by Google
• High-Level Thesaurus (HILT) Project - Cross-searching of various controlled vocabularies and terminologies.
• Library of Congress Subject Headings (Library of Congress) - LCSH as linked data using SKOS
• LibraryThing JSON Books API
• LibraryThing Web Services API (LibraryThing)
• Mendeley - Academic reference manager and academic social network web site.
• Metadata Crosswalk Service (OCLC, Experimental) - Transforms metadata from one format to another.
• OpenCalais (Thomson Reuters) - Creates rich semantic metadata for the content you submit
• Open Library
• OpenURL Gateway (OCLC) - delivers "Where Are You From?" resolver services via portable and institution-independent OpenURL links
• Oxford English Dictionary
• pewbot (Huddersfield University Library) - "People who borrowed this also borrowed that" service
• Scopus (Elsevier) - Search this massive journal article database
• Talis Platform
• Terminology services (OCLC) - Various controlled vocabularies, including LCSH, MeSH, and TGM
• ThingISBN (LibraryThing) - Takes an ISBN and returns a list of ISBNs from the same "work" (i.e., other editions and translations)
• ThingLang (LibraryThing) - Takes an ISBN and returns the language of the book
• ThingTitle (LibraryThing) - Takes a title and returns a list ISBNs from the most likely LibraryThing "work," the LibraryThing title and a link to the LibraryThing work page
• Virtual International Authority File (Library of Congress (LC), the Deutsche Nationalbibliothek (DNB), the Bibliothèque nationale de France (BnF), and OCLC)
• WorldCat Basic API (OCLC) - Citation access to the full WorldCat database that anyone can use
• Worldcat Identities (OCLC) - Get information about people/organizations listed in Worldcat
• WorldCat Registry and detail (OCLC) - Find and retrieve basic information about institutions and consortia profiled in the WorldCat Registry, not limited to OCLC members
• WorldCat Search Service (OCLC) - The full API to the largest bibliographic database in the world - some restrictions on who can use
• xISBN (OCLC) - Find all the ISBNs plus edition and other metadata related to a work
• xISSN (OCLC) - Find all the related ISSNs
• xOCLCNUM (OCLC) - Find all OCLC Numbers plus edition and other data related to a work

Libraries

• Cambridge University Library - A suite of APIs for a variety of services (library catalog, institutional repository, etc.)
• Mirlyn (University of Michigan)
• North Carolina State University

Directories

• Information Environment Service Registry (JISC)
• ProgrammableWeb.com - Both library and non-library services.

Tools & Specifications

• Atom Publishing Protocol
• DLF ILS Discovery Task Group (DLF-ILS) Technical Recommendation (Digital Library Federation) - "An API for effective interoperation between integrated library systems and external discovery
  applications."
• Document Availability Information API (DAIA) Perl Module
• IndexData Software Tools
• Jangle - An open specification for exposing content (starting with, although not exclusive to, library services) consistently and simply using the Atom Publishing Protocol.
• OpenSRF (Evergreen)
• PatREST and more (including modules to interface w/III) (John Blyberg)
• SRU: Search and Retrieve via SRU (Library of Congress) - Next generation library search and retrieve protocol
• Z39.50 (NISO) - Legacy search and retrieve protocol

Note: My thanks to Owen Stephens for the original list.