I can admit I probably wouldn't have rated this so high if I didn't love the series and it wasn't a re-read. It is good, and the mystery is well-crafted, but what really made this reread great was what I already knew putting the book in context.
For example, I knew

that when Bertie refused to admit why he needed the bathroom, it meant Stephen decided to have celebratory sex with his boyfriend after being told his murder attempt was successful.

Definitely made the reading experience better.