Looked this up to answer a question about imayo songs on the SCA:Japanese FB page. Noting here for future reference:
Yung-Hee, Kim Kwon. Songs to Make the Dust Dance: The Ryojin hisho of Twelfth-Century Japan (Berkeley, Univ. of California Press, 1994) ISBN: 9780520080669 Link is to LEGAL electronic copy of full book. (via Lisa Joseph/Mistress Saionji no Hana)
Konishi, Jin’inchi _A History of Japanese Literature, Volume 2: The Early Middle Ages_ (edited by Earl Miner, trans by Nicholas Teele) (Princeton, NJ; Princeton University Press, 1986) ISBN 978-0691101774 has some good discussion on the subject. Konishi had some interest in the subject and devotes a chapter to it. Note–be sure and get the SECOND volume. The first one just has a few lines on imayo, and while the third does has some bits and pieces on it, the 2nd volume is where most of the information is.
Malm, William _Japanese Music and Musical Instruments_ (Tokyo; Tuttle Publishing, 1990) ISBN 978-0804816489 devotes a little time to the subject and is also just good reading about Japanese musical structure anyway, although I suspect he got his imayo info from Konishi’s work? Neither of these works are recent, however.
Miller, Stephen D. _The Wind from Vulture Peak: The Buddhification of Japanese Waka in the Heian Period_ (New York, Cornell East Asia Program, 2013) ISBN 978-1933947662 has some translated imayo and looks some at structure.
I found some cites on a JSTOR search–mostly articles by Yung-Hee Kim Kwon (who wrote _Songs to Make the Dust Dance_ that Lisa Joseph already mentioned), but there were two articles that looked like they might have be of interest to you based on conversations we’ve had in person (this was to the original poster, who lives here in Calontir): Meeks, Lori. 2011. “The Disappearing Medium: Reassessing the Place of miko in the Religious Landscape of Premodern Japan”. History of Religions 50 (3). University of Chicago Press: 208–60. doi:10.1086/656611.
Goodwin, Janet R.. 2000. “Shadows of Transgression: Heian and Kamakura Constructions of Prostitution”. Monumenta Nipponica 55 (3). Sophia University: 327–68.
Online article about a lecture in 2015 by Dr. Elizabeth Markham, who is currently researching imayo Songs of Peace: On Japanese Imayo of the 12th Century.