Howgego's Ancient History from Coins

The most popular e-Book reader is the Amazon Kindle. I haven't used one. Only Seven numismatic books are available in Kindle format.

Christopher Howgego's Ancient History from Coins is available in Kindle format, and also in Microsoft Reader and Mobipocket format, Sony eBook format, and Adobe Reader format.

Why is this Howgego's book on ancient numismatics so widely available in electronic format? Almost no other books on coins are available electronically, but this one is in every online e-Book store.

I read it years ago, in dead-tree format, and thought it was pretty good. I no longer have a copy.