I'm about to publish my work in progress. Is there anything I need to know? Do I need to state that it is unfinished or is it assumed due to it being published with you?

Good luck with your book! You can set your book completion percentage here:

Author > Books > [Book Title] > Book Web Page ? Book Status

The percentage you set will be displayed under your book's cover image on its Leanpub web page.

It's also a good idea to mention that the book is in-progress in the About the Book section, with some information about your intended publication schedule; this gives potential readers confidence that you'll finish.