Thanks to all who responded to my query. I received four recommendations of Vit Bubenik's two books:

1996. The Structure and Development of Middle Indo-Aryan Dialects. Delhi: Motilal Banarsidass.

1998. A Historical Syntax of Late Middle Indo-Aryan (Apabhraṃśa). Amsterdam and Philadelphia: John Benjamins. 

Additionally, one correspondent mentioned the 1965 English edition of Jules Bloch, Indo-Aryan from the Vedas to Modern Times. I note that this version, which was translated by Alfred Master, who was in correspondence with Bloch until the latter's death in 1953, is a substantial revision of the original 1934 French edition and takes account of Bloch's own corrections and notes.

with best regards,

Matthew