A Talent For Murder, Peter Swanson

After a whirlwind romance, quiet librarian Martha married Alan. But when she thinks she sees his mask slip, she starts to fear that his frequent travels might be a cover for something far more sinister. As she secretly starts to map his movements she unearths a string of dead women, but could these two things really be linked? Unsure of her own instincts, Martha turns to an old friend, Lily Kintner, who once helped her escape a toxic relationship in grad school.

But what Martha doesn't know is that Lily has a dark side of her own . . .