Wow. After the irritation I felt with the main guest character in the last one, wow. Beautifully judged and balanced. I loved the way Bob's emotions have shorted out so much by the end that he lapses into report-speak. That's very psychologically real. And the bleak black humour. 
 I was totally wrong-footed as to who the big bad was, and that's a thing that surprises and delights an old lag like me. Having spent five books getting to know this cast, the shocks in the last few pages are going to take some time to bed in. Looking forward to #6