Volume 32 of RMA Monographs has just been published:
Anna Zayaruznaya, Upper-voice structures and compositional process in the Ars nova motet, published in May 2018.

It can be ordered from Routledge, and all earlier volumes of RMA Monographs are available to order, with the regular Routledge discount to members of 30 per cent. The whole series is listed here.