Superlative treatment of fly fishing, with an emphasis on approach and presentation rather than "matching the hatch."
I'm not sure if this is a good "first" book for someone who wants to learn about fly fishing. There are certainly slimmer, more easily digestible how-to books out there that assume less prior knowledge and will get the reader into some fish quickly (the Orivis beginner's guide comes to mind). But this is an excellent second book. And if this is the only other fly fishing book you buy, you'll probably be better off than most anglers.
Reading this book is a bit like having an extraordinarily knowledgeable uncle take you under his wing to teach you the everything that he's learned about fly fishing after years of experience. Lefty spends a tremendous amount of time focusing on small details that don't get enough attention. How often do you hear people tell you to keep your hooks sharp (and show you the proper way to do it) if you're missing strikes? How many books focus on the minutiae of every different type of cast you would want to make? How many offer helpful suggestions such as to go one line weight lower than normal on order to make your presentations more delicate and catch more fish? Not too many, I'd guess. Lefty focuses on these important details concerning presentation and tackle that will result in many more hookups than you would otherwise get.