Wikidata - thoth-pub/thoth GitHub Wiki

Wikidata, an open knowledge base, has a dedicated project to encode book metadata, WikiProject Books. The website lists all definitions of appropriate metadata categories based on the Functional Requirements for Bibliographic Records

Metadata & API

Wikidata relies on Wikibase as

software that enables MediaWiki to store structured data or access data that is stored in a structured data repository. Source: Programmer's Guide to Wikibase

The Wikibase API is a RESTful web service, with demo apps and code snippets in Python, Javascript and PHP demonstrating how to use various modules of the MediaWiki Action API available on GitHub/Mediawiki.

The API endpoint is accessible via https://wikidata.org/w/api.php

Another relevant project is WikiProject Source Metadata.