@amberwingart IP-free books are books for which the copyright is expired and the material is then in the public domain. The copyright lenght varies from country to country - there is a wiki article about it:
https://en.wikipedia.org/wiki/List_of_countries'_copyright_lengths
Here is a list I found online:
The Project Gutemberg website also has mostly IP-expired titles, but not exclusively, so one hast to look a bit deeper.
If you choose a book to illustrate and publish, I would definitely research that particular title to make sure that you are free to operate with commercial gain.
This suggestion, by the way, was advanced by one of our faculty here at SVS in one of their videos - I think it was Jake Parker in the video "You need a product, not a project".