http://www.amazon.co.uk/exec/obidos/...238721-8922813
The above is a link to most of the books in the Sharpe series. Sharpe's Eagle is a good one to start with as it was the first written (but not the first chronogically).
I think a weapon such as a catapult could have been useful in disrupting the charges we see in the film. They would have to be very large and quick to reload to make a big difference though. I imagine that the horsemen would be able to see these weapons being fired and have time to react to them. It would however ruin the desired effect of the charge.