As long as a book has no sales, you can easily delete a book.
To delete a book, select Author in the menu at the top left of a Leanpub page. Then navigate to:
Books > [Book Title] > Admin > Delete Book
If your book has sales and you want to remove it from the Leanpub store, the best thing to do is go here:
Books > [Book Title] > Admin > Unpublish Book
There, you can unpublish your book.
Then go here:
Books > [Book Title] > Admin > Stealth Mode
On that page, you can put your book in "Stealth Mode", which means your book will not be viewable on the Leanpub website.