Experience
I agree that Royce's Sailing Illustrated is a hugely worthwhile book.
It isn't the knowing of the terminology that will make you a valuable crew member; it's the cogent application of the knowledge you have acquired. Sailboat handling and racing skills come over time, as you learn from practical experience what works and what does not. Theory is one thing. Hands on experience is what really counts, and often there is neither the time nor the opportunity to have much of a discussion about relative merits.
I've been racing for about 30 years. The last time I raced my boat, in 2007, I had some very good people on board. On one leg, a boat not in our class was attempting to go over the top of us, meaning that it would take our wind, and the kid who was handling the main just reached over and pulled the main up with his hands so that we could sail higher and shut the door on the faster boat, forcing him to go through our lee. It was so simple, so quick and so effective that I am still in awe of what he did. I'm a fairly good driver, but need reminding from time to time to keep my mind on what I am doing. With this crew, sailing a 1973 27-foot Ericson, we beat the second place boat in our class by 6:24 in real time. That was in a 12-mile race. Good crew members are worth everything. They are money in the bank. Maybe some day you'll become as talented at this 16-year-old kid. It's a lofty goal to shoot for. Good luck.
Morgan Stinemetz