A new book, Patrick Bowen's A History of Conversion to Islam in the United States:
White American Converts before 1975
(Brill, 2015), does not deal directly with Traditionalism (though Guénon is mentioned) but does deal with various topics that are part of the background to Traditionalism, and part of the background to Sufism and Traditionalism in the United States. There's lots on the Theosophical Society
, on Martinism
, on early neo-Sufis
(if not exactly perennialism) and on changes in attitudes to religion
in the nineteenth century. A good book, recommended.