An excellent book. The author begins by exploring how people really talk, then moves on to examine how we need to adapt this when writing fictional dialogue, to retain the essence of realism without the boredom factor. He goes on to look at the different styles of dialogue, from realism and heightened realism to the highly stylised dialogue written by people such as Oscar Wilde. Fascinating stuff, with gems of relevant info on every page.
Unlike many books on fiction writing, the author does not talk down to the reader. Neither does he include general fiction-writing info which is not strictly relevant to the title of the tome - a huge bonus, since it can be irritating when every fiction-writing guide you pick up, whatever its professed subject matter, tells you the same fundamental stuff you've read a thousand times before. Full marks for sticking to the subject and covering it in depth, rather than padding it out with "general" writing tips.
I strongly recommend this book.