The sailor can read every sail trim book written about increasing sail trim performance and the sailor won't know what the author is talking about.
I might have mentioned this before, but here goes again.

I've read ALL the sail trim guides and Gentry and other online sources.
NONE of them does what Don's excellent Sail Trim book does.
None of them.
It is, by far, the BEST reference for the why, what and how to trim sails I have ever read, and I've been reading about this subject for 60 years.
I rarely use the word "best" when it comes to sailing, because we all (should) know that all boats are compromises when it comes just about everything. There have been many respondents on various boating forums who have correctly responded to those ubiquitous "What's the Best...?" questions. We've all seen them.
But when it comes to SAIL TRIM, Don's is THE BEST.
As a handy dandy extra bonus, there is a great section on racing that I have never seen anywhere else, ever, never. It alone is worth the price of admission. Think about it: How come there aren't a whole lot of "Teach me how to race?" questions on this or other boating forums? You know, like the recurring "I want to buy my first sailboat and go around the world but I've never been on a boat?" ones. I maintain that part of it is that a successful racer doesn't want to give away his techniques to the competition. I sure don't. But part of it is also "feel", something that one can't impart to anyone else. That said, Don's explanations and ideas in his succinct few pages on this subject alone are priceless and well worth the read.
For those of you who would like to get the most out of your SAILboats, please buy Don's book. It will help you to do so.