I am looking for an API where I can search for a book and retrieve a list of books related to it. It would also be useful to have this kind of search for authors.
The Spotify API has a function just like this, but for musicians.
Project Gutenberg has what's called a Bookshelf, which are categories.
Also, for each title, there is a link called "Also downloaded"
So for book:
the related titles would be:
As Joe mentioned, "relatedness" is pretty subjective. I have found that this often means one of two things. The first is related based on similar terms, say in the description of the book. The other is based on previous user behavior. For example, if your user is looking at a book, you might want to show, "people who read this book, also read this one".
For the first type, many search engines offer a function called "morelikethis" which allows you to find related items. You would simply upload your books to the search engine and then make the API call to find the similar items.
For the latter, and assuming you wanted to look for related items based on your own set of books, you would need some user logs along with the book data. Once you have this there are a lot of great options for finding related items. If you are interested in learning how to do this, I have a github tutorial here, which shows how to do this (and create an API) with Movie data.