For more than you'd ever want to know about sail trim, written clearly and illustrated well I think the best book is the North U. TRIM
I've used it to teach a few classes
, and in the process have looked at virtually every book mentioned so far. TRIM is in my opinion the clearest while also being the most comprehensive.
While the TRIM book is oriented to racers, the same principles apply for cruisers. The only thing it lacks is content related to cruising-specific rigging
It's $25 when ordered directly from North but for some reason is $182 on Amazon. Weird.