Amongst his most noted publications is Dictionary of the Place-Names of Wales jointly with Richard Morgan (Gwasg Gomer, 2007 and reprinted in 2008).
Ymhlith ei gyhoeddiadau amlycaf yn y maes mae Dictionary of the Place-Names of Wales ar y cyd â Richard Morgan (Gwasg Gomer, 2007 ac ailargraffwyd yn 2008).