Wraiths and Changelings (1978)
My review:
Really rather dull Mitchell. The first half (the expedition of ghost-hunters, and the telling of ghost stories) is mildly entertaining, but the second half, consisting of routine alibi-breaking, is dull—and Detective Chief-Inspector Rees does not have Inspector French's eccentricities or witty dialogue to enliven the procedural. Dame Beatrice herself is of no use whatsoever, as the murders are solved by Rees—the only reason for her involvement that I can see is to have Laura's daughter Eiladh married off.