Ambassador Stephen Seche was the American ambassador to Yemen between 2007-2010 and he has written extensively on the Houthis of Yemen and I consider him to be one of the best sources about the Houthis. Here is a document that appeared at the WikiLeaks website that he wrote in 2009 which was marked secret and the title was:
Tehran’s Shadow Looms Large, But Footprint Is Small
https://wikileaks.ORG/plusd/cables/09SANAA1662_a.html