What: Lethal Marriage by Nick Pron
Why: The book that is probably one that is probably most critical in understanding the degree of physical, psychological, emotional and sexual abuse Homolka experienced with her psychopathic sexual sadist boyfriend/husband. Lays an important foundation to everything that came next.
What: Deadly Innocence by Scott Burnside and Alan Cairns
Why: slightly different sources, good book, adds further details about things from all sides
What: The Galligan Report
Why: Judicial report by Justice Galligan who was commissioned by the Harris government to do an inquiry on the wildly unpopular "Deal with the Devil' struck by the previous government. It's dense and legal at times, but it allows a MUCH more accurate understanding of what happened in Homolka's plea deal, and why. It debunks popular beliefs like the erroneous and oft repeated "fact" that when the tapes were found it was revealed that Karla had lied about her involvement. If you like reading CANLii documents, you'll like this too.
https://drive.google.com/file/d/0B3ORC8Ev2VnnaDg5LWVWVjJzUGM/view
What: Invisible Darkness by Stephen Williams
Why: Long, extensively sourced and ambitious. It's essential reading on the case BUT you can't start with it because he's dismissive of a lot of the context and when it comes to interpreting Karla's behaviour it becomes almost novel-like. He's overly suspicious in general and you won't have an appropriately complete and nuanced view of the big picture if Williams' views become your basic framework for understanding the case. So do read this, but start somewhere else.
There are a few other recommendations I could make for reading - some of the 2005 court proceedings for example, and an early book by an academic named Frank Davey called Karla's web which focuses on the media's role in warping the understanding of the case. But it gets pretty esoteric at this point and probably more than anybody needs to know about such a soul crushing case.
Hope you enjoy your reading if you take it on!