The Master of Haengma
Rayheart (1k), This book is very good, have a lot of problems of haengma connections, and normally the answer isn't very easy, in many cases the haengma isn't the first move, thing that make it very enjoyable and good to study. The problems are more dificult when you are finishing the book.
Slarty (2d) Excellent content on a subtle subject, not found in many other books, afaik. The layout is beautiful, and the examples are top notch. The text is not what you'd call proofread English, but it can actually help you develop a vocabulary for evaluating exchanges. Any sdk might find the ideas accessible enough, but fluent reading is a major plus.
Chapter 1 Patterns of haengma Chapter 2 Practicing Haengma Chapter 3 Amateur Haengma Chapter 4 Professional Haengma Chapter 5 Comparing Haengma Techniques
to be added