"Essays on Mysticism and Java" by Paul Stange
The best link to know about Sumarah history and spirit.
"Javanese Mystical Movements"
During the last decades Javanese mysticism has become more and more of interest to anthropologists. They base their books, articles, doctoral theses, etc. partly on Dutch studies during their colonial past, partly on their own observations during field-work. Java is particularly fascinating because its culture bears traces of various religions.
"Sumarah: A Study of the Art of Living" by David Howe
A PhD dissertation (1980) and a useful description of Sumarah that can be found on his website http://thebookofbeing.net/index2.html.
However readers should be aware that the rest of the web page is about other personal experiences very different from Sumarah tradition.
Wikipedia page in Bahasa Indonesia for Sumarah