The Monmouthshire Antiquarian Association (formerly called the Monmouthshire and Caerleon Antiquarian Association) have published a number of free standing monographs, some of which can be found online:
Morgan, Octavius & Wakeman, Thomas – Notices of Pencoyd Castle and Langstone. Newport, MCAA, 1864.
Mitchell, Elizabeth Harcourt – The crosses of Monmouthshire. Newport: MCA, 1893.