Here is a book for people who would like to improve their skill in observing, recalling and describing faces. It is called “Mug Shots A Police Artists Guide To Remembering Faces”, by Douglas P Hinkle.
In brief, it provides an excellent, systematic approach to observing faces and in chapter nine offers a self-training plan for the reader. It’s not terribly long, but is easy to read and very informative, in my opinion. One important recommendation that he builds on is that it is best to prioritize your observations. People tend to be distracted by changeable features, such as clothing, hair style, etc. He teaches how to first focus on features that are the least likely to change, working from features that tend to be most permanent to those that are more changeable.
There is much more to like about this book. If you want to improve your observational skills for faces, I recommend you check out this book. Just for the record, it is a very easy read. I discovered the book this morning and read the bulk of it in a few hours.
The copy I read was available online through archive.org. I would prefer to purchase a copy to own, but so far only found one at a ridiculously high price. The book was initially published through Paladin Press, but they closed down awhile ago, so I have a feeling the book may only be available second hand.