Someone got here by googling "How to cite chapbook in MLA format." Well, anonymous friend, chapbooks are really just books, except shorter and cheaper, so you'd cite them like a book. Just that it doesn't have an ISBN number doesn't mean it's not a book. Do you think Gutenberg and Caxton gave a flying fuck about ISBNs?

Anyway, as an example, if I were to cite a poem by John Sakkis' The Moveable Ones, it woud be like this:

Sakkis, John. The Moveable Ones. San Francisco: Transmission Press, 2007.

But don't take my word for it. I just used this measly example to plug my friend John's chapbook. But I hope it helped.