If there is any type of plagiarism in a book (a few sentences perhaps) and they are not cited properly, then it is plagiarism. If an "author" is using another author's work in any way, then the "author" has to have written and signed permission. Ebook version or print, it's the same. If saw what you saw, then I'd summarily contact the publishers of the original work and report it. If the "author" has acquired written and signed permission, then there's nothing to worry about. If not, then you will have helped to make sure that the original author retained the copyright of his/her labor.
I know at least two of them are still living. The stories seem to have been lifted directly from the website (with only names changed). The erotica stuff is a bunch of stories written in the 60s and 70s. I found the site where they seem to have been pulled as well. I suppose it doesn't hurt to contact the site owners themselves. If it is legit they will either let me know or decide for themselves.