What an API actually is
An application programming interface lets one program request information from another system in a structured format rather than reading a search page by hand.
Open Library is an Internet Archive project maintaining an open, editable catalog of books.
What a title search returns
Matching titles, authors, first publication years, editions, Open Library identifiers, ISBNs and other identifiers, subjects, cover references, and availability information, at both work level and edition level.
Courteous use
Open Library currently permits one request per second for unidentified traffic and up to three per second when the application identifies itself with a user agent and contact information. Cache results, use it for human facing discovery, and use monthly data downloads for bulk analysis.
What it does not replace
Open Library provides one bibliographic layer. Comprehensive title clearance also uses other library catalogs, retailer databases, search engines, trademark searches, domains, and social platforms.