API.
There isn’t a public API on this site.
The library you keep here belongs to the account saved in this browser. Covers, prices, and catalog records are read when you open a page that needs them. They are not offered as a feed or a key you can call from somewhere else.
If you need a record the screens don't show, write to mail@2manybooks.com and say what you actually need. The documentation covers how the product itself works.