Cracks me up that OP is trying Anna's Archive before Wikidata, NGL! Both great sources, though.

I recently (a year ago... wow) dipped my toe into the world of library science through Wikidata, and was shocked at just how complex it is. OP's work looks really solid, but I hope they're aware of how mature the field is!

For illustration, here are just the book-relevant ID sources I focused on from Wikidata:

  ARCHIVERS: 
  Library of Congress Control Number    `P1144` (173M)
  Open Library                          `P648`  (39M)
  Online Computer Library Center        `P10832` (10M)
  German National Library               `P227`  (44M)
  Smithsonian Institute                 `P7851` (155M)
  Smitsonian Digital Ark                `P9473` (3M)
  U.S. Office of Sci. & Tech. Info.     `P3894`

  PUBLISHERS:
  Google Books                          `P675`  (1M)
  Project Gutenberg                     `P2034` (70K)
  Amazon                                `P5749`

  CATALOGUERS:
  International Standard Book Number    `P212`
  Wikidata                              `P8379` (115B)
  EU Knowledge Graph                    `P11012`
  Factgrid Database                     `P10787` (0.4M)
  Google Knowledge Graph                `P2671` (500B)

Mind pointing me to an example book in Wikidata? I managed to find a few, but not if I search by ISBN, which makes it hard to find.

Unless you mean the fact you can find the identifier for a book in several different websites in there, in which case, I did find it.

Not gonna lie, I didn't even know Wikidata existed until now. I'll look into it today and create a ticket for a new extractor.

Thanks for letting me know!