I have many books on this subject, but this is by far the best. It is lavishly illustrated throughout, with page after page of delightfully competent examples by Maughan, demonstrating every aspect of the skill.
Maughan begins by explaining his methods and preferred materials. The Carbothello pencils he uses are wonderful to draw with. He moves on to explain chiaroscuro, value, form, shadow and so on, all of which I recommend you read thoroughly before taking on his extensive chapters on the subject of drawing the head.
He deals with the subject in ample detail, and to his credit he writes very well too. There is a lot to read but every word of it is useful. His examples (and there are very many) are not the unusably rough pieces found in other books, but are in large scale, often in close-up and annotated, so you can really see in detail what he's doing and how he does it. He also rather cleverly provides equally high quality examples of how NOT to draw - for example, showing what happens when the proportions are incorrect or the lighting is poor. You could use this book to merely ape the Maughan style, but I believe it has much more value than that if you take the time to understand his methods and apply them to your work.
Often with such books, the benefit you derive from them depends to some extent on whether you like the artistic style of the author. I happen to like it, but even if you don't, Maughan's tremendous skill and understanding of his subject is such that it would be a shame to miss the opportunity to learn from him. This book has improved my skill greatly.
Even if you don't draw, the book makes an excellent coffee table piece.
I've had my copy from Amazon for while, but at time of writing (21 Mar 2006) the price is an absolute bargain at £12.89.