I think she was the first author I'd read that so articulately demonstrated what a sociopath is capable of with "The Stranger Beside me" -- that Ted Bundy was capable of giving what seemed to be compassionate support working suicide hotlines, to being a calculating law student* and serial killer.
I think her true crime novels opened up a lot of eyes as Americans matured out of the naive "free love" social cultures of the 60's and 70's. She's one of the major authors that lured me to being a true crime buff, I feel like a lot of her writings were a public service in raising awareness about stalker type crimes and pathologies.
RIP, Ann, and thank you for your huge body of work, so much to learn from!