At Leanpub you hold the copyright: you can publish your book on Leanpub while you are shopping it around. Getting traction on Leanpub can help you build buzz and strengthen your negotiating hand with publishers. For example, you can ask to keep the ebook rights, since the Leanpub royalty rate of 90% minus the 50 cent flat fee is much better than what most publishers offer.
To 'retire' your book from Leanpub, just go to the 'Book Admin' tab on the 'Actions' page for your book, and click the 'Retire my book' button. You can unretire your book later, if you decide to start selling your book on Leanpub again.