The strongest/most interesting parts of the book are Campbell's stories of medieval Scottish history and her descriptions/perceptions of the natural (sometimes supernatural) world around the castle. She relates well what it was like to walk in two worlds - the historical and the present and of her growing awareness of the rift between the two.
If I were directing MacBeth, this would be one of the source books even though Cawdor was never MacBeth's castle. Campbell captures the bloody battles and competition amongst the highland warrior clans explicitly - including a tale about Muriel, the only woman ever to be Thane, who was branded as a baby (strictly for identification purposes in case of kidnapping) and whose nanny bit her little finger off above the joint (when the feared kidnapping finally took place, just to be absolutely sure, should the girl ever return home that she was indeed the heir to Cawdor). Campbell's chronicles of her father's erratic behavior, addictions, and abuse along with a very long list of eccentric ancestors makes great material for creating the characters of MacBeth and Lady M.