This book does a really good job of explaining ways to identify how far features of the face should be apart. It's has given me several tools in this regard. It's also pretty good at giving you an idea of how to sketch out a proportional and mostly accurate portrait quickly. When it comes to likeness, expression, and more in-depth details, this book is pretty lacking.
The book spends a lot of time setting up the process of drawing and how it relates to portraiture. There's a really nice walkthrough of how to work generally through each feature of the face in a quick manner, and as stated above, the tools for identifying the width, space, and length of each feature and their positions are really helpful. When it gets to the finer details and the greater complexities of the face, it looks more and more like notes from a sketchbook rather than anything actually instructional. In reference to likeness, the message is basically "don't worry about it" and then "focus on what you find interesting about that person's face." When it came to expression, it was more vague advice.
The first half of the book is definitely worth a read-through, the second half not so much. Check it out from your local library.