What does "book post" mean?

The system and regulations in which books and printed matter may be sent through the mail, often at a reduced price.

What is a synonym for "book post"?

Common synonyms include: airmail, correspondence, express, frank, letter, mail.

noun

noun ·Rare ·Advanced level

Noun

1
The system and regulations in which books and printed matter may be sent through the mail, often at a reduced price. dated