“The Christian Instructor”
Charles' Welsh Catechism, "Yr Hyfforddwr, was originally published in 1807.  This English translation by his grandson, Rev. D. Charles, Aberystwyth, was originally published in 1867.
  Also available as PDF and scan.
 
“An Exposition of the Ten Commandments; by Way of Question and Answer”
Originally published in Welsh in 1801, this English edition was translated by Thomas Charles himself and was published in 1805.
Also available as PDF and scan.
 
“Yr Hyfforddwr yn egwyddorion y Grefydd Gristnogol”
A gyhoeddwyd yn wreiddiol yn 1807.  Cyhoeddwyd yr argraffiad hwn yn 1901.
Diolch i Eifion Jones am y ‘PDF booklet’.
Also available as
PDF and scan.