These are only three books that I have read on the subject of drawing portraits that I can recommend. While there are many out there I found most leave out valuable information. The books below deal with two different methods of drawing the head, which you chose will depend on the method that you will want to use.
NOTE: This list is based off of the books that I own / have read and therefore does not cover all of them. It is also based on my opinion of these books yours however may be different.
Artists complete guide to drawing the head – William L. Maughan
This book uses chiaroscuro which is a method of using values to depict the face. This book has a good explanation of the proportions of the face and how to use light and shadow. It also breaks things down into a step by step process and continues on to show you how to use the same method to create portraits from multiple sources and also has a small section on colour. Even if you are not intending to draw using only value the information in this book is very valuable.
Drawing the head and hands – Andrew Loomis
This book shows you how to draw a portrait starting with a basic frame and building upon it. Not only does this book show you how to draw adults (both male and female) but also explains the differences between people of various ages, something I have not seen in another book on the subject. As the name suggests this book also includes a section on how to draw hands although the main focus is on heads.
The artists complete guide to facial expressions – Gary Faigin
This book starts out explaining the bones and muscles of the face and then proceeds into explaining the changes that take place with various facial expressions. This book is filled with plenty of images to help explain what happens to the face through various expressions. For anyone considering drawing portraits I recommend they get this book or one like it so they can accurately portray expressions in their drawings.