I watched the TV documentary of this, many years ago.
Mary Ann Cotton murdered around 20/21 people before she was caught and hanged. She was only ever convicted of the one murder, though. She murdered anybody she felt the need to and that included children and these were mostly her own. She poisoned them.
As well as the relatively short story of her life David Wilson tries to explain why it is she is not as well known as Jack the Ripper. Well, Jack the Ripper committed his crimes in the capital city, he eviscerated his victims (as opposed to poisoning) which caught the public's imagination and he was never caught, so no closure. Mary Ann committed her crimes in the North East, moved around a lot, poisoned her victims - which, at the time, looked very much like Typhus Fever and Typhoid to a Victorian doctor - so none of them were immdediatley obvious let alone as blood thirsty as evisceration and she was caught and hanged, so closure.
The book is well written if a little dry and Wilson spends too much time comparing her relative none-fame with others who were famous. Maybe we just didn't want to know.