This handy little book provides a great historical introduction to Orkney. It gives a brief historical outline of each period in the islands' (very long) history together with details of the sort of sites and monuments which survive from said period, and then a gazetteer of the actual sites and whether or not they are accessible to the public.
It is probably a better buy, in that respect, than Historic Scotland's "The Ancient Monuments of Orkney" (being much more comprehensive) and would make an excellent companion to the masterful, must-buy "Orkney: An Illustrated Architectural Guide" by Leslie Burgher. Only downside: no colour photos. But then other books fill that niche.