While down in Cornwall this year, I looked out for any archaeology gazetteers to Scilly. Here were three I found:

"Visitor's Guide To Archaeology in Scilly" - Luke Over, illust. J.F. Crussell (A.H. Read & Son, 1974). Although some of the archaeology is a little outdated, this is a nice little A5 size guidebook, divided into an overview of general history and then an island by island gazetteer, with suggested trails for exploring.

"Ancient Monuments of the Isles of Scilly" - Dept of the Environment Official Handbook, B.H. St. J. O'Neil (originally published 1949, third ed 1983). A chronogical overview, 36pp, b&w illustrations/photos.

"Scilly's Archaeological Heritage" - Jeanette Ratcliffe & Charles Johns (Historic Environment Unit, Cornwall CC 2003). Excellent little guidebook, 55pp. General overview and then island by island gazetteer. £4.50 from the Tourist Info in Hugh Town, St Mary's.