Susan Cooper’s series “The Dark is Rising” has two books set in Cornwall, England. One is “Over Sea, Over Stone” and the other is “Greenwitch”. They are in the fictional town of Trewissick, which is modeled after the real town of Mevagissey. This photo is from Susan Cooper’s Facebook page, giving a sense of the vibe of the place.

According to Wikipedia,

Mevagissey (/ˌmɛvəˈɡɪzi/; Cornish: Lannvorek) is a village, fishing port and civil parish in Cornwall, England, United Kingdom. The village is approximately five miles (8 km) south of St. Austell. The parish population at the 2011 census was 2,015, whereas the ward population at the same census was 4,354.

A possible version of Trewissick, according to Calmgrove on WordPress, in their post “Treasure at Trewissick” https://calmgrove.wordpress.com/2016/01/13/oversea/

Calmgrove also adds this detail –

“Trewissick seems to be a conflation of two locations: one is Mevagissey, southeast of St Austell, which Cooper knew well from holidays there; the other is Trevissick Manor, between Mevagissey and St Austell, currently offering farmhouse accommodation. Much of the action of Over Sea, Under Stone can be located at Mevagissey — Chapel Point could be Kenmare Head with its standing stones, Penmare headland is where local legend sites gravestones, St Peter’s church is St John’s in the book and so on. Mevagissey also has a Feast Week with a carnival, as does Trewissick in the book, though this takes place at the end of June as opposed to August, as in Over Sea, Under Stone.”