The world building of a London a hairs-breath removed from our own, in a time just after the First World War is unobtrusive and perfectly constructed. The plot is excellent (seriously, I had plot-envy).
Saul and Randolph are the perfect couple, men so broken that they aren't sure they can trust their judgement, or indeed are worthy of the others regard and affection.
There were moments of, not terror per se, but suspenseful chills that made me scared to breath for fear that something bad would happen if I dared.
And to counter that there were moments within the relationship that were an absolute delight where I found myself smiling at the book like an absolute loon.
I can't recommend this highly enough.