I don't think a book has to necessarily be old to be rare.
I have a hard copy of the very first "Jehovah's Witnesses Alternatives to blood transfusions" 1973. Printed and compiled by "prominent" Canadian JWs and later published by the WT's Hospital Information Services. The first hospital "manual" that was used by the JW Canadian pilot group for the now named HLC.
I know....It's a book that wasn't circulated to the general public - medical professionals were targeted with that publication. Medical evangelizing done by JWs.