The question seems to be very broad and therefore difficult to answer satisfactorily. The enquirer may wish to provide more detail of what is being sought.

There is a very wide range of sources relating to Catholic History in the British Isles and 'Massing Priests' was often used as a derisory title in reference to the recusant priests in the period after the break in Henry VIII's time with the Catholic Church, and particularly in the later periods of religious and political persecution of Catholics in England in 16th-18th Century.

There may be a single book with the words 'massing priests' in the title but I have not heard of one such.

You might direct the enquirer to the Catholic Records Society for a more informed reply. The most recent address I hold is: Catholic Records Society 12 Melbourne Place Wolsingham Co DURHAM DL13 3EH

Phone 01388 527747

My own search on the internet produced the details of the book below in case it is of use.

DIMOCK, N. [Ed.].:
  Missarum Sacrificia. Testimonies of English Divines in respect of the claim of the 'Massing-Priests' to offer Christ for the quick and the dead to have remission of pain or guilt.
  Eliot Stock, London, 1896.. pp. 246. Bookseller Inventory #28
  Price: £ 15.00 (Convert Currency)
  Bookseller: Brian Carter, Deddington, OXF, United Kingdom

Peter Harrison
Clifton Cathedral Guides
E-mail : fisherpeteruk@yahoo.co.uk