Thirteen Diamonds. Lillian Morgan, #1

Lillian Morgan, a retired mathematics professor who can remember any number but not what she did yesterday, is resigned to living a boring life when Gerald Weiss, a Nobel-prize-winning Economist and fellow resident of Silver Acres Retirement Community starts choking and then dies during a weekly meeting of the bridge club. Gerald was holding a bridge hand of 13 diamonds when he died, which is rarer than winning the lottery.
When the autopsy reveals that his death was caused by an extreme allergic reaction to shellfish in what was supposedly a tuna casserole served by the bridge club lunch committee, Lillian decides to investigate the possibility of murder. With the help of her best friend, Lillian finds out that two of the four members of the lunch committee were more-or-less girlfriends of Gerald; there may even be senior sex involved.
A third member of the lunch committee owed Gerald money and the bridge hand may be a fraud. Lillian's investigations almost get her expelled from Silver Acres, but she persists, coming close to getting herself killed in the process.