What is a Chapbook?

Wikipedia On Modern Chapbooks:

Chapbook is also a term currently used to denote low-cost hard copy production, particularly of poetry. The genre has been revitalized in the past 20 years by the widespread of low-cost copy centers and the cultural revolutions spurred by both zines and slam poetry, the latter generating hundreds upon hundreds of self-published chapbooks that are used to fund tours across the United States and, in fact, the world.

More on the history of chapbooks at Wikipedia

Another site with historical information on chapbooks, The Scottish Chapbook Project

In short, chapbooks are:

Slim, soft-cover poetry books, inexpensively produced. Mine is 27 pp. and in this case is not self-published, although the precursor to this one, which was a bit shorter, was self-published.