If you're looking for a book about Kowloon Walled City I would recommend City of Darkness (revisited) by Ian Lambot and Greg Girard. There is very little in here about the Walled City and a lot of what is written seems faulty from what else i've read.
The book contains many many stories of people who have got over addiction and physical ailments through Jackie Pullinger's work and through God. It's hard to believe any of them when they all read so formulaically. "X person was addicted to X drug for 15 years, X person prayed with other believers, X person's physical ailments miraculously healed, X person felt at peace as they had never done before, X person cried tears of joy, X person now believed in Jesus."
I believe Jackie Pullinger did a lot of amazing work while in Hong Kong and clearly many of these people's lives were dire and that so many ended up in such awful situations is harrowing. I am not trying to tarnish her faith, but this book was bad.