Currently, all you can do is redirect a domain to a specific page (using your own DNS provider), since once someone is on Leanpub, they often buy multiple books, and it would be really confusing to people if they didn't know they were on Leanpub and then ended up with a Leanpub account.  (Also, you wouldn't want to do our support work for purchases, either of your books or of other authors' books!)