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
"We must accept our burdens gracefully, for when the student takes the Path, he is often plunged into ranges of experience which other men escape, in order to teach him to have command of himself in his environment, and thus learn the reason why he has been placed there."