Yuzu Nembutsu?

I am interested in learning more about this sect of Japanese Pure Land Buddhism. However, contrary to what is available on Jodo Shu, Jodo Shin Shu and even the Ji sect - or Ippen, at least, I have found almost nothing. There is a smattering of info in The Foundations of Japanese Buddhism by the Matsunagas, but otherwise I have found on fragments of info on the Internet.

Just a chance, but has anyone out there come across anything else?

I dont know how helpful the following on Ryonin will be.. it also seems to reference the Matsunagas material.

http://books.google.com/books?id=ErMRGiNcxJIC&pg=PA613&lpg=PA613&dq=...

Peter, thanks. It didn't take me very far, but I appreciate the suggestion.

River of Fire, River of Water by Taitetsu Unno is excellent.

Yes, it is a great book, but it does not touch on the Yuzu Nembutsu which is what I am looking for more info on.