Do not use The Castle. It was previously a prescribed text in the Retreat from the Global module. I'd hazard to guess that it was pulled off because too many kids were analysing it in too much of a simplistic manner. One of the objectives of the extension course is to promote independent research and the selection of original supplementary material. As The Castle does not fill either requirements, markers may feel as if you are "douple dipping".
Erin Brockovic would be a good text; the ones I used were Tim Winton's Time and Tide (feature article) and The Rabbits, a picture book by Shaun Tan and John Marsden. If you would like to look at an essay I wrote that received 25/25 before my HSC, please PM me!