I have just visited Fridd-Gatheiniog and Hendre-Meredydd, part of the Hendre-Ddu 'complex'. My first visit to this valley and my anticipation of things of interest was rewarded! A quick half-hour pottering around the old slab mill and the pit of Hendre-Meredydd has whetted my appetite to explore further later in the year.
For a quite detailed history of the quarries and tramway, with plenty of maps, diagrams and illustrations, I recommend 'The Mawddwy, Van & Kerry Branches' by Cozens, Kidner and Poole, pub. Oakwood Press, ISBN 0-85361-626-4
Patrick