There is a very effective branch of yoga that few people practice - Nyasa Yoga. Nyasa Yoga is a physical yoga internal purification and cultivation practice. It is related to an entire host of yogic exercises for clearing your nadis of obstructions by invoking energies said to "divinize" a practitioner's body. The practice diffuses energy into the body by associating mantras with every part of the body and the whole of it. For the best results you need to stay on each spot to sufficiently energize the prana of that region, and visualizing the targeted muscles helps open up the nadis quicker. When you recite a specific mantra from that location, you should linger and try to feel a full vibration in the area. The result is significant progress towards cultivating a "deva body" (astral body double) that can leave the physical body at will, which is the goal of many yoga texts.
Basically, Nyasa Yoga stimulates the body's prana - which is your own latent Qi, spiritual energy, vital energy or life force - into moving and then fully opening your nadis. Through methods that involve mantra and concentration, you generate and then lead your prana from place to place to open various sections of your body, and to create a "shield of divine protection" for the body as a whole. Together with other yoga exercises, Nyasa Yoga results in your channels being purified and a subtle body being generated, which is the first stage of the spiritual path equated with enlightenment. Many Nyasa and related yoga exercises are described that rarely appear in print.
Inside are various Nyasa Yoga practices as well as sectional body cultivation techniques from a number of different traditions. There are also discussions on special (and sometimes secret) mantras and holy places in the world that are excellent for cultivation. Special instructions on Mantrayana practice, Yoga Yajnavalkya exercises, how to open up your spinal channels, the real location of chakras in your body, what happens during the twelve-year period of kundalini awakening, how to cultivate the five koshas, and methods to specifically cultivate feminine energy are all revealed as well as a number of other rare and interesting topics.
If you wanted just one internal energy ("nei-gong") cultivation book for yoga practice then this is a veritable treasure trove of cultivation techniques that even includes esoteric Taoist and Buddhist Vajrayana meditations that do not require special empowerments.