I believe Showell Styles lived in llanrwst, and passed away back in 2005.
His 1974 publications, on the five classic locations in the National Park, included the Carneddau, The Glyders, Snowdon, The Moelwyns, and the Rhinogs.
Each book had great descriptions of each area, from every points of the compass, and even today, make great reading, and have not dated one little bit, as the walks described are the same now, as they were back in the early 1970s.
It would not surprise me that John Gillham, got his inspiration for writing his four pocket guides, from Showell Styles.
The publications are very similar in the way they introduce visitors to each outlying area, even though there's a huge 37yrs dividing each writers works, they are very similar in nature, and more than encourage a new walker to visit NW Wales.