A couple of articles on Isis at SDJournal (free access)
A couple of articles on Apache Isis have just been published by the Software Developers Journal, available for free download.
The first article is a general introduction to Apache Isis, while the second looks in detail at Isis' RESTful APIs. Both were written by Dan Haywood, Isis' current PMC chair.
To download the journal, you do need to register, but it won't cost you anything other than your email address.