I have much enjoyed John R. Kenyon, ‘Kyffin Williams and the guidebooks to the four great Edwardian castles of North Wales: a bibliographic essay’, The Castle Studies Group Journal 27 (2013-14) 241-44. These cover the castles of Beaumaris, Caernarfon, Conwy, and Harlech. My copy is a second impression (1966) but the originals date to 1961. The guides (‘illustrated souvenir’) were written by Alan Phillips and ‘produced by the Information Branch of the Ministry of Public Buildings and Works’. Phillips also prepared a souvenir guide to monasteries in North Yorkshire. He seems to have studied at University College, Oxford.
The four guides replaced the old Office / Ministry of Works guides to Beaumaris (by Wilfried J. Hemp, 1933), Caernarfon (by Sir Charles Peers, 1929), Conway Castle and Town Walls (by A.J. Taylor, 1956), and Harlech (by Hemp and Peers, 1927).
Williams’ covers were replaced by line drawings of the castles in later editions of the guides. My copy of Phillips’ guide to Harlech is the Department of the Environment (‘on behalf of the Welsh Office’) edition of 1974. The guides both contain fold-out plans and tours of the two castles. The DOE guide to Harlech has a description of the castle in Welsh printed inside the back cover.
The new MPBW guides mark a move away from the ‘Blue Guides’ to more heavily illustrated booklets such as the combined volume for Stonehenge and Avebury.
Heritage Futures 
Do you also have images of Williams’ designs for the guidebooks for Conwy and Harlech castles? Note that the Conwy and Caernarfon guides are up for sale on eBay, should you be interested (not my own listing!).
Thank you for this information.