For smaller mines and trials not covered in Bick's books, the following book will be found very useful: J.R.Foster-Smith "The Mines of Montgomery & Radnorshire" (British Mining no.10), Sheffield: Northern Mine Research Society, 1978.
If you are really serious, then this book will also be useful: O.T.Jones "Memoirs of the Geological Survey: Special Reports on the Mineral Resources of Great Britain: vol.20: Lead and Zinc: the Mining District of North Cardiganshire and West Montgomeryshire", London: HMSO, 1922, reprinted Sheffield: Mining Facsimiles, 1986.
Both reasonably often available from specialist mining history booksellers with online listings.