Firebase used to be excellent dataset, based on "semantic web" design model (RDF etc)
company acquired by Google and absorbed into search results.
The original open dataset is migrated to Wikidata, that is now main free, structured data resource.
Freebase was a large collaborative knowledge base consisting of data composed mainly by its community members. It was an online collection of structured data harvested from many sources, including individual, user-submitted wiki contributions. Freebase aimed to create a global resource that allowed people (and machines) to access common information more effectively. It was developed by the American software company Metaweb and run publicly beginning in March 2007. Metaweb was acquired by Google in a private sale announced on 16 July 2010. Google's Knowledge Graph is powered in part by Freebase.
|Current status||Offline (since 2 May 2016), succeeded by Wikidata|
Wikidata is a collaboratively edited multilingual knowledge graph hosted by the Wikimedia Foundation. It is a common source of open data that Wikimedia projects such as Wikipedia, and anyone else, can use under the CC0 public domain license. Wikidata is a wiki powered by the software MediaWiki, and is also powered by the set of knowledge graph MediaWiki extensions known as Wikibase.
Wikidata is a free and open knowledge base that can be read and edited by both humans and machines.
Wikidata acts as central storage for the structured data of its Wikimedia sister projects including Wikipedia, Wikivoyage, Wiktionary, Wikisource, and others.
Google Books is effort to make book content more discoverable on the Web. Using the Google Books API, your application can perform full-text searches and retrieve book information, viewability and eBook availability. You can also manage your personal bookshelves.
ISBNdb.com: used to be free, from Amazon, not free anymore
Amazon: The API you need is the Product Advertising API