What is a chapbook?
Merriam-Webster defines a chapbook: a small book containing ballads, poems, tales, or tracts.
Writer's Digest offers a more detailed explanation:
https://www.writersdigest.com/whats-new/what-is-a-poetry-chapbook#:~:text=A%20chapbook%20is%20a%20small,(Common%20poetry%20terms.)
