The Puzzle of Ethics, Peter Vardy - There's a chapter on Aquinas, Natural Law and Proportionalism. As always, Vardy writes very clearly. This chapter would be easier to digest with subheadings, though!
AS/A2 Philosophy of Religion and Religious Ethics for OCR: Study Guide Bob Bowie- There is just under a page on Natural Law. This book can only act as a revision guide or overview (which it is very good at)
Ethical Studies (Second Edition) Bob Bowie- A much more thorough coverage than the above - 13 pages, including an evaluation and some useful extracts. I really like this book: each chapter includes 'What you will learn bullets, key questions and chapter summaries.