Koormayoga refers to the practice of withdrawing the senses like a 'Koorma' aka tortoise withdraw its legs into the shell. This withdrawing of senses is very important for a yoga sadhak at least in the initial stage. Koorma avatara reminds us that ones journey for moksha (amrutha) starts with this withdrawing of senses. Can you please recommend a book on koormayoga?
Koorma is a metaphor and not a specific branch of yoga. What you are asking about is Raja Yoga. Swami Vivekananda said (Complete Works, V7, p 71; available here - http://www.advaitaashrama.org/cw/content.php):
Control the mind, cut off the senses, then you are a Yogi; after that all the rest will come...You continually do it unconsciously as when your mind is absorbed; so you can learn to do it consciously.
Read his translation of Raja Yoga in Volume 2, same link, it includes his translation of Patanjali's Yoga aphorisms.
Another book is Sri Sankaracarya's Aparoksanubhuti translated by Swami Vimuktananda. A good step by step instruction.
But, to make real progress a teacher, guru, is needed. All the books say that a teacher is needed. This is really the first step. Without it, you are depending only upon your own ego to guide you, and it is the blind leading the blind. Do you try to learn astrophysics on your own? Or medicine? Or engineering? Why do people think that to truly unlock the mind a teacher is not needed???