The International Standard Book Number (ISBN) is a numeric commercial book identifier that is intended to be unique.
Building on work from Grimme et al. 2019, the COPIM WP5 Scoping Report notes that ISBNs
"are certainly not PIDs and multiple ISBNs can exist for the same work, print, digital, vendor editions etc., but as noted above, publishers continue to rely on them instead of the DOI. Grimme et al. comment “that ISBNs were designed as retail identifiers” and this creates an issue for OA as there is no incentive for a retailer to distribute the OA version because there is no sales commission on the OA version. This is despite evidence to suggest that there is no impact on sales, and in some cases sales more increase." Scoping Report, 2021
Publishers purchase or receive ISBNs from an affiliate of the International ISBN Agency.