• There is an entry for him in Ian Maxted's *The London Book Trades 1775-1800* (1977), and--
• Harry Carter comments (favorably) on the Pliny in the *Catalogue of the Edward Clark Library* (1976), vol. I, p. 145. [Bill Peterson said that the catalogue includes history of printing but is not well known and is findable.]
• James Mosley may well know more about him.
• Dennis Bryans suggestions:
  -There is a bit about peripheral printers around Caslon and Baskerville in Johnson Ball's William Caslon Master of Letters, this is a book overladen with much genealogical detail, but frustratingly not much of it seems all that relevant to research into fine printing. Have you tried searching through literature on the Earl of Stanhope's career?
• -Stanley Morison's Four Hundred Years of Fine Printing and found Richie's Pliny - Epistolae (1790). Very restrained. Quite nice. I wondered, of course, who J. Sammells was.    
• Then I looked up my copy of Bewick's memoir (mine is the Selwyn Image edition) but found no reference to Richie, but I wondered. Bewick travelled a lot, to London and to Edinburgh, so the printing and crafts people that he mentions, like Bulmer, might suggest something to you that I am missing. Then there is Morison of course. I have scanned his book on Bell, but haven't read it thoroughly. The copy I have access to is in an institutional collection with all that that entails. Still Bell is of the period, and Morison would definitely have known more than he put into the Pliny caption.
• Lasly, I consulted a book on Baskerville and came across numerous references to Aris's Birmingham Gazette. This publication by Samual Aris appears to have discussed matters typographical in some depth. It might be worth a look.