Kyffin was the author of Rhann o Psalmae Dafydd Brophwyd Ivv Canu ar ôl y dôn arferedig yn Eglwys Loegr (printed in London by Simon Stafford for T[homas] S[alisbury] , 1603 ). It is doubtful whether this booklet was ever published; the N.L.W. copy is the only one extant, as far as is known. It contains only the first twelve psalms and the first five verses of the thirteenth, set to the metre known as awdl gywydd , side by side with the Biblical text and following Kyffin 's epistle dedicatory giving the reasons which prompted him to undertake the work. In view of what Morris Kyffin says in the preface to Deffynniad Ffydd Eglwys Loegr , 1595 , regarding the need for a metrical version of the psalms in Welsh , it is not improbable that it was at his instigation that his brother Edward undertook the task. Thomas Salisbury , in a letter to Sir John Wynn of Gwydir , states that Kyffin had composed about fifty metrical psalms before he fell a victim to the plague in 1603 .

In one only of the three variant forms of the preliminary pages of Psalmae y Brenhinol Brophvvyd Dafydd … by Wiliam Middleton , 1603 , there appears, on pp. 3-4, an elegy to the author, ‘ Mawl-gerdh farwnad i gapten William Middleton ,’ by Kyffin . It is not certain whether he or another of the same name, who graduated at Oxford and was vicar of Whitford and of Caerwys at the beginning of the 17th cent. , is the Edward Kyffin who is named as the author of ‘ The pronunciation of the Letters in the British tongue ’ together with ‘ A comparison of the pronounciation of the letters in Welsh, to the Greeke and Hebrew letters ’ which appear at the beginning of Car-wr y Cymru, Yn annog ei genedl anwyl … , 1631 .