Two years ago I published a book on the Merrifield family 'From Devon to Australia: A 19th Century Narrative' it is based on the unpublished letters of the Merrifield family.
I am the great-great-great grandfather of William Merrifield (librarian at Tavistock). His eldest son Samuel came to Australia in 1854 and the story is based on 114 original letters sent to him by his father and brother John (Jack) from 1855-1911. There were a small handful of letters still existing, which Samuel sent home. I know Samuel's letters were distributed amongst his brothers and sister, so I don't know whether you have come across any in the family archives.
I have had 4 visits to Tavistock to do my research and have, through contacts with distant relatives and the use of the letters, archival materials, primary source records, photos etc was able to write a comprehensive story about the life of the Merrifield family from early 1800s to the mid 1950s.
I followed the male line in the book, and am aware that William Merrifield (1870-1938) married Rosa Tapp (1875-1960) in 1899. As far as I could ascertain William and Rosa had two children (both girls - Daphne and Rosa). This is where my research concludes for this line, because I did not follow the female line once they married. Too hard and I would never have finished the book.