I suppose in all fairness I was warned: the back of A Most Extraordinary Pursuit concludes "piecing together the strange events of the days before his disappearance, Truelove will discover the folly of her misconceptions--about the whims of the heart, the motives of men, and the nature of time itself...," which I took as the sort of hyperbolic drivel one is apt to find on the backs of novels. I was wrong. I love mysteries, I love science fiction, but when the time-traveling villains showed up 3/4 into what I had taken to be historical fiction I just got mad. That, as it turned out, was just the beginning in a series of increasingly improbable plot-unfoldings, and a casual reference to Einstein and 'more things in Heaven and Earth' and lines like "You will possibly not believe me, when I tell you what came next. I should not have believed it myself, if I encountered the incident in a book or play..." or, worse, "Science and religion both rose up and forbade the very idea; such a brazen act had no place in a logical universe," did not help.
All the other ingredients were right where they belonged, of course: the unattached spunky first-person narrator, the charming cad with whom she trades zingers in their game of Romantic ping pong, an adventure-mystery to exotic locations in a period setting. I even went cheerfully along with the ghosts, which I thought were brilliant, and the out-of-body experience, which was daring. When I squinted, I could imagine Elizabeth Bennet, and sometimes Peter Wimsey.
What Williams does well, I think, is world-building - she writes highly detailed, sensorial fiction with sights and sounds and smells - occasionally too obtrusive for my taste, but it is a matter of taste, and I respect that it takes a lot of research and is hard to do.