The CONTENTdm API is a read-only web service to access CONTENTdm collections and records in JSON or XML format. For a given CONTENTdm website URL you can access the API via the /bl/dmwebservices/ URL path. For example, on the CONTENTdm Sandbox site, the JSON call to list the collections looks like this:
https://sandbox.contentdm.oclc.org/digital/bl/dmwebservices/index.php?q=dmGetCollectionList/json