Nightingale calls them the fae but that's a catch-all term like the way the Greeks used the word 'barbarian' or the Daily Mail uses 'Europe'. [loc. 260]
So off Peter goes to the surprisingly unmagical countryside. He speaks to the parents of Nicole Lacey (who had an imaginary friend, Princess Luna) and Hannah Marstowe: he encounters some of the local watercourses: he talks to the retired wizard (who provides more backstory on Nightingale's past) and to his granddaughter, who has a way with bees. He's joined by Beverley Brook, who encourages him to face up to the grief and rage he's been suppressing. And he solves the case -- for values of 'solves' that include 'works out what happened'. We don't get to find out why it happened.
As a Rivers of London book I think I'd find this slightly disappointing: two of my major interests (Nightingale and London itself) are absent. But as an admirer of Tana French's In the Woods, I was fascinated: this novel, in some respects, was the inverse of that one, and though it has a happier ending and a great deal more overt magic, the same sense of brooding fear is there.
Also, Princess Luna? Yay!