We offer many books, courses, audio lectures, and articles about yoga. Here are some to get you started.

"The word YOGA comes from the root Yuj which means to join, and in its spiritual sense, it is that process by which the human spirit is brought into near and conscious communion with, or is merged in, the Divine Spirit, according as the nature of the human spirit is held to be separate from (Dvaita, Visishtadvaita) or one with (Advaita) the Divine Spirit." - Swami Sivananda, Kundalini Yoga
"The word yoga means in general to join one's mind with an actual fact..." - The 14th Dalai Lama
"Yoga is a Sanskrit word that means “union.” It is describing the union of our experience with reality. It means that our individual perception becomes unified with facts, with what is real, with what is true, not what is merely illusion [maya] but what is real. So, that is really what yoga means: it is the experience of uniting with reality. From that perspective, you can see that doing different bodily positions [i.e. Hatha Yoga] is not going to have much impact on your perception or consciousness, it is only going to change the position of the physical body. We need to change the position of our perception." - From the course Practical Spirituality
"Those who are addicted to sensual pleasures or those who are arrogant and proud, dishonest, untruthful, diplomatic, cunning and treacherous and who disrespect the Guru, Sadhus and elders and take pleasure in vain controversies and worldly actions, can never attain success in Yogic practices.” - Swami Sivananda
Accomplishing Dream Yoga
Dalai Lamas: Sexual Yoga
Dream Yoga
Dream Yoga Course 01: Awakening in Dreams
Dream Yoga Course 02: Dimensions and the Mind
Dream Yoga Course 03: Analyzing Experiences
Dream Yoga Course 04: Interpreting Experiences
Dream Yoga Course 05: Returning to Experiences
Dream Yoga Course 06: Symbolism of Dreams
Hatha Yoga
Hatha Yoga Pradipika: Retain the Semen
Hymn to Kali: Laya Yoga
Krishna: the God of Love, Sex and Yoga
Kriya Yoga
Naropa: Dream Yoga
Naropa: Inner Heat Yoga
Naropa: Karmamudra Yoga
Naropa: Yoga of Psychic Heat
Oordhvareta Yoga
Patanjali: The Sexual Basis of Yoga
Practical Spirituality
Raja Yoga
Sexual Yoga
Sivananda: Tantra Yoga
The Divine Mother: The Power of Love, Sex and Yoga
The Foundation of Dream Yoga
What are Dream Yoga and Astral Projection?