Supposedly the definitive text for bastles. Part Two by McDowell and Mercer covers bastles and describes in detail 89 bastles. Richly illustrated with photos and plans.
However, see the criticism made of the choice of the term 'bastle' to describe relatively mean farm houses made by Philip Dixon (Shielings and Bastles: A reconsideration of some problems (1972), Towerhouses, Pelehouses and Border Society (1979) and, most recently, (2013), 'Border Towers: A Cartographic Approach' in Jeremy Ashbee and Julian Luxford (eds), Newcastle and Northumberland Roman and Medieval Architecture and Art (British Archaeological Association Conference Transactions XXXVI).