After Williams's death some of his collection was compiled into "The Iolo Manuscripts" by his son, Taliesin Williams. His papers were used by many later scholars and translators, and for reference by Lady Charlotte Guest as she translated the prose collection "Mabinogion". She did not, however, rely on William's editions of the tales themselves, except for "Hanes Taliesin". Later still, further Williams forgeries were published in a text known as "Barddas". This work, published in two volumes in 1862 and 1874, was claimed to be a translation of works by Llywelyn Siôn, detailing the history of the Welsh bardic system from its ancient origins to the present day. Though it contains nothing of authentic Druidic lore, it is the fullest account of the mystical cosmology Williams developed. Other works by Williams include the "Druid's Prayer", still used by the Gorsedd and by neo-Druid groups, a treatise on Welsh metrics called "Cyfrinach Beirdd Ynys Prydain" ("The Mystery of the Bards of the Isle of Britain"), published posthumously in 1828, and a hymn series published as "Salmau yr Eglwys yn yr Anialwch" ("Psalms of the church in the wilderness") in 1812.