On this page you will find some interesting links towards Ericofon sites as well as a few books that are available. Amongst this information there is a lot more out there if you look carefully.
This website from Richard Rose (www.ericofon.com) contains loads of information that was (and still is) for me the best source for reference.
As far as I know there are only 2 books on this phone available when it comes to the history. The first book is the one from Bob Mills (1997) and seen as the first written history on the Ericofon, seen from an early Australian perspective. It can be purchased here from Richards site.
Peter Schröder from Germany wrote a newer version around 2010 with much more details, more accurate information and less assumptions. The latter is still available as CD in both German and English and a real asset for the collector. You can order it at email@example.com
There are many other sites on the web that are not specifically on the Ericofon itself but have an article or post on it.