[wredcoll]

There is absolutely not such an api. For one thing, nobody actually knows because nothing stops random people from publishing a book at any time.

For another, the major book publishers like amazon want to prevent anyone else from having their data.

Beyond that, book metadata is probably the least standardized thing I have ever seen in my entire life.

Titles, subtitles, editions, editors vs authors, series...

[Scarblac]

I thought ISBN numbers might be somewhat centralized. But I don't know anything about the domain.

[wredcoll]

The short answer is that isbns are sold more or less like ipv4 addrs. Companies buy a block and then some get assigned to books.