Now her novel about the rehabilitation of a child murderer has won the UK's top prize for teenage fiction.
Looking for JJ, published by Scholastic Press, portrays Jennifer Jones emerging on licence from protective custody with a new identity at 16, having killed her best friend when she was 10.
"The big question I wanted young people to think about was: can we forgive people for what they've done, and can they forgive themselves?", said Ms Cassidy, a former secondary English teacher who has written 18 crime novels for young adults, covering topics such as stalkers and drug dealers.
"I could have been a lot more sensationalist, but I wanted to show an ordinary girl from a not very supportive home, who had a chance to be OK but because of the way a particular day turned out, did something that she will probably never be able to explain."
Ms Cassidy taught in east London schools for 11 years before becoming a full-time writer in 2000 and continues to teach creative writing and GCSE English at Redbridge institute of adult education.
Adam Robbins, English teacher and head of media studies at Archway school, Stroud, Gloucestershire, was on the panel of judges (including five young readers) that awarded Ms Cassidy the Booktrust Teenage Prize for the best novel for 12 to 16-year-olds.
Since then she has also been shortlisted for the Whitbread Children's Book of the Year prize.
Mr Robbins said: "It has the exciting plot and pace that younger readers would like and enough subtlety and sophistication for older readers.
"You would find yourself feeling sympathy for someone who had murdered a child and be surprised that you felt like that."
More details about the prize and the shortlisted books on www.bookheads.org.uk; www.annecassidy.com