I really enjoyed Whispers Under Ground, but felt there was a tad of something missing. I expected MORE Nightingale, more conflict, more Rivers, more pace. I love Peter's "easy" first person narrative. 
 This is good by any other authors standards, and is compulsive reading, but Ben Aaronovitch set the bar so high with the first two books, this didn't quite reach their standard for me. BUT I really hope there is a 4th and we don't have to wait too long for it.