It's possible that Harper Collins, who put out the current paperback editions in 2010, doesn't have the rights to do electronic versions and they reside with his estate.
An email to his son might bring an answer...
http://trentzelazny.com/index.php?page=contact
I'd think the publisher would be willing on books that have sold enough to be kept in print since the late 60's.
My guess is that with patience they will be coming.