Del Rey Books don't give out authors' email addresses. However, they do say that if you send them a snail mail, they will pass the letter on. The current mailing address for Del Rey is:
Del Rey Books
NY, NY 10036 USA
If writing to David and Leigh, it's a good idea to use the two-envelope method. Write your letter and place it in a stamped envelope with 'David and Leigh Eddings' written on the front with no address. Put this envelope into another envelope which you mail to Del Rey Books at the above address.
If you want a reply, and you live in the US then adding a third, self-addressed, stamped envelope will make that more likely.