As of winning the D+T prize this year at school I have been given 12 pounds to spend on a book. Any good Suggestions ?
A bit over your £12, but this is the bible:
http://www.amazon.co.uk/Aerohydrodynamics-Sailing-C-A-Marchaj/dp/0713650737 (http://www.amazon.co.uk/Aerohydrodynamics-Sailing-C-A-Marchaj/dp/0713650737)
"High Performance Sailing" by Frank Bethwaite is a pretty standard text these days. I used to enjoy reading "Boat Tuning for Speed" by Imhoff & Pranger. It was published in the 1980's so a bit out of date now but probably the same principles still apply.