I have a copy of Grinsell's The Archaeology of Exmoor (1970) and found it useless in my quest. He only mentions a few of the many stone settings and gives no exact locations.
Much better is EH's The Field Archaeology of Exmoor (2001) which does give grid references for 2 stone circles, 8 stone rows, 57 stone settings and 37 single or paired stones on Exmoor.
Burl's From Carnac to Callanish (1993) is worth consulting for the rows and some settings.
I've not been able to find a copy of Quinnell and Dunn's Lithic Monuments within the Exmoor National Park (1992).

I like to do my research when planning a field trip and, to be honest, I've not come across any stone settings described as a square, apart from the quincunxes which have a fifth stone in the centre.