I read a lot of mysteries - they make up more than half of my reading and most of my fiction reading. I also re-read some of them, which probably sounds odd to others. If you know who "dunnit" there's no reason to read it again, right? Not really. I enjoy trying to solve the puzzle, but if the only thing the book has going for it is a good puzzle, it's barely worth reading the first time. A mystery novel is first and foremost a novel, and therefore needs to be well written, with vivid descriptions and interesting characters. And sometimes, after 15 years, I may forget the killer's identity and motive.
Susanna Gregory's A Plague on Both Your Houses introduced Matthew Bartholomew, professor of medicine at Michaelhouse College, Cambridge. I've now known Matthew for 15 years (10 in his timeline), and it was interesting to see how he's changed in that time. He was fairly new to the University in 1348, somewhat unsure of the politics, and in love with his roommate's sister. Overall, he seemed a bit, innocent, perhaps? Matthew is not a cynical character, but he seemed a bit less accustomed to the backstabbing (literal and figurative) so common to both town and gown. Matthew's two closest friends, his book bearer Cynric and theology instructor Brother Michael have also changed over time. The Cynric of 1348 is a vaguely drawn character, little more than a shadowy figure who appears when most needed and not the trusted and perceptive man with a sense of humor I encountered in later books. Brother Michael underwent an even greater transformation - through most of the book, Matthew didn't know whether he could trust his gluttonous colleague who had not yet revealed his background as a courtier. Now that I've reminded myself of who these characters were, I want to re-read the entire series and once again watch their growth.
As I read, also I discovered I'd forgotten as much of the plot of A Plague on Both Your Houses as I had of the characters' earlier personae. I remembered that the plague came to Cambridge, and that Matthew contracted it and recovered with the help of one of his students, and that he was informally engaged to his roommate's sister. I'd forgotten that the book started with the death of Michaelhouse's master, and that Sir John had been murdered. Michaelhouse's new head is the vain and bombastic Wilson, and during his installation, someone murders another Michaelhouse member. Matthew begins his investigation, but is soon interrupted by the spread of the plague. Somehow, through his exhaustion and frustration, he manages to stumble onto the plot at the root of the murders and, with help from an unexpected person, solves the mystery.
I don't know if this is intentional, but Gregory brings up an immunological issue. Michaelhouse's laundress Agatha claims to be immune to the plague, and she appears to be right. Brother Michael also avoids developing the disease, despite multiple exposures. HIV researchers have found that some people are strongly resistant to the virus, and that the mutation that protects them from HIV may have protected their ancestors form the plague. I hadn't heard that when I first read this book (maybe the theory hadn't even been known outside of professional circles), so it really stood out when Michael mused why he seemed to be immune.