Who is Fetullah Gulen, what is the Gulenist movement, and what are they up to? (Claire Berlinski)
Actually, Claire Berlinski researched and wrote a piece about these subjects back in 2012. It's a useful introduction, even though (or perhaps because) when you finish reading it you will probably feel that a great deal about Gulen and his movement remains hazy, uncertain, mysterious, and perplexing.
"Who Is Fethullah Gülen?
Controversial Muslim preacher, feared Turkish intriguer—and “inspirer” of the largest charter school network in America"
P.S. One interesting American angle in this story is the way that the Gulenists seem to have successfully used charter school programs in the US as a vehicle for getting US taxpayers to (indirectly, and in effect) subsidize the Turkish operations of an international Islamist organization with a strong political agenda. A clever and impressive feat.