The Internet Archive, along with some researchers have found an odd loophole that lets libraries copy (scan) books that are out of print. This may be worth remembering if people are looking for some between-the-wars material that is out of print.
https://blog.archive.org/2017/10/10/books-from-1923-to-1941-now-liberated/
The “Sonny Bono Memorial Collection”! Librarians never forget.
Ironically, some of these works may still be under copyright in NZ, as here copyright continues for 50 years from the death of the author/creator.