The best books on the subject of Subtle Energy, available at Amazon. (Your purchases will help to support our work!)

The definitive Bench Yoga manual. Develop both strength and flexibility. In particular, develop the specific combination of strength and flexibility you need to sit comfortably, effortlessly. Learn to use the breath, both to make the practices more effective, and to experience a joyful energy-release that transitions easily to meditation.

Yoga for Everyone
For beginners, intermediates, and experts. Everything you need to know to get the most out of your Yoga Meditation Bench. Develop a strong sitting posture. Build your capacity to sit in advanced positions. Grow to Lotus. Discover ways to accelerate your asana practice, and stimulate internal energy (chi) flows.

Moola Bandha - a small, yet thoroughly comprehensive treatise on the subject of the incredibly powerful perineal lift—arguably the most important of the Subtle Energy activators. 
The Yoga System of Health and Relief from Tension—A slim volume that delves into the inner world of Yoga, in addition to the physical poses. Quite illuminating!
The Art and Science of Raja Yoga—A large, comprehensive volume that covers Ananda yoga practices, in addition to internal-energy practices. Has the first good explanation for what "gaze at the third eye" really means. (When that statement is understood as the all important Orbital Lift, the practice becomes as powerful as Moola Bandha, in its own way. (And when the two practices are combined, the results are spectacular.)