Google Books
Overview
Usable by default: Yes
Crawls metadata: Yes
Crawls covers: Yes
Overall quality: Good enought as a baseline
Overall quantity: Almost perfect. Has at least some data for almost every book.
Longrunning: No
Description
Gets data from Google Books API. It's a good source for metadata and covers. It's very much not perfect, but it's good enough as a baseline. It's also very fast.
Configuration
This source doesn't need any configuration. It's usable by default.