Hope I can be of some help..

Botterel and Hunt both wrote tales of folklore and legend in the early 1800's. In the mid 1800's Wilkie Collins visited the area and wrote "Rambles beyond Railways" about his travels, including a visit to Trethevy Quoit. From information gained during these trips he wrote the novel "Basil", it is based in West Penwith and the finale takes place on the coast at Porthcurno and at Treen Castle. I haven't been able to read the book yet (has anybody out there?).
I have read RM Ballantyne's "Deep Down", published in 1869. It mainly concerns mining in the St Just area but it has great discriptions of wandering over the wildmoor where the old stones sit.
I am currently ploughing my way through several novels by Sir Arthur Quiller Couch (grandson of Dr Jonathon Couch, naturalist and amateur archeoligist, he was responsible for early excavations on the barrows at Pelynt). Quiller-Couch, or Q as he was known, was writting the novel "Castle Dore" when he died, Daphne du Maurier kindly finished it. I am sure I will find refferences to ancient sites in the other books as he also likes to write about wild moorland and tales from the past.
Du Maurier of course wrote Jamaica Inn, I have only read one of her books. "The House on the Strand" which is about 13th century monks and mind altering drugs so I don't know if she features ancient sites in any of the others.
The Cornish poet, John Harris, was born on the side of Carn Brea above Redruth. His poems give great discriptions of graggy tors and ancient stones.
Sorry if I have not really tracked down any specific sites but give me a day or two and I will see what I can do.