Hi Stumped and welcome to the site.
Eric is a bit of a strange beast as far as the books go as it was originally published by a different publisher to Terry's other books. That other company own the rights and therefore decide what format to publish in. Eric was, originally, an illustrated book - hence the different publisher.
However, a quick look on Amazon.com shows that Eric will be available
for Kindle - although it doesn't look like it is yet.