An analogy
Leaves of Grass
A posthumous book is one published after the author has died.
Book 3 is called THE CROWMASTER, and it is published in February in the UK. Book 4 is called DOC MORTIS, and it is published in August.
Jacqueline Wilson's first published book was called love lessons
R. Duane Gilliam published a book called "The South" in 2020.
people today know about Marco Polo's adventures because Rustichello's book about Marco Polo's adventures was published in different languages, and is in English.
harveys book is called EMBRYOLOGY, and was published in 1651
David Pelzer
what is the book called that benjaminfranklin published each year.
Terry Pratchett's first book, "The Carpet People," was published in 1971 when he was just 23 years old.
This is called a transcript. Writers send the transcript to a publisher (with legal protection to prevent them stealing ideas) in hope of getting it published into a book.
The book you are referring to is likely "PostSecret" by Frank Warren. It is a project where people anonymously mail in their secrets on a postcard which are then published in book form.