The best, most accessible book explicating Patanjali's famous Yoga sutras. Author Christopher Isherwood does an excellent job of explaining the multi-thousand year old sutras in language and metaphors that make sense to a modern Western reader. Also, the Sanskrit and arcana of ancient Indian/Vedic culture are kept to a minimum so you get the important concepts w/out the mass of historical details that tend to be of more interest to scholars than lay readers.

If you do yoga and want to understand your practice better or have a general interest in what might be the oldest technology/techniques of consciousness, I highly recommend you acquaint yourself w/Patanjali and recommend Isherwood's book as a highly readable and approachable introduction.