In the yogic tradition there is karma yoga, bhakti yoga and jnana yoga. Karma yoga deals with karma, bhakti yoga deals with devotion, and jnana yoga deals with buddhi, the mind. Everyone has to deal with their own mind and with their own life, but first you deal with karma and bhakti. The books written about these yogas always start, “Now therefore I teach you karma yoga (or bhakti yoga, or jnana yoga).” This is the first line in every one. ‘Now’ means that you have fulfilled all the conditions. Now that you have fulfilled all the conditions, I will teach you bhakti yoga. Now that you have fulfilled the conditions, I will teach you jnana yoga. So, in order to learn jnana yoga and practise jnana yoga you have to fulfil certain conditions, otherwise it will only be intellectual.
Intellectual knowledge is not experience, it is information. If you have read about a particular sweet but not eaten it, you may have information, but you have not experienced it. Experience is personal knowledge, so jnana yoga is personal knowledge of the self. For that you must have a peaceful mind, restrained senses, disenchantment with worldly pleasures, faith in guru and God, and know these are the conditions.
The six conditions required by a jnana yogi are sama (calmness of mind), dama (restraint of the senses), uparati (indifference), titiksha (endurance), shraddha (faith) and samadhana (mental equilibrium). If he does not fulfil these conditions, but just goes through the Upanishads and the Brahma Sutras, then it will only be intellectual knowledge, not an experience. That is called jnana yoga.
Swami Vivekananda, Swami Sivananda and Yogi Aurobindo have written very good books on jnana yoga. The most important text on jnana yoga is the Brahma Sutras, aphorisms on Brahma. It should be studied after having studied Panchadashi, a book of fifteen chapters. The Brahma Sutras is the text book of jnana yoga, just as Patanjali’s Yoga Sutras is a text book on raja yoga, just as Shandilya Bhakti Sutras and Narada Bhakti Sutras are text books on bhakti yoga, and Mimamsa Sutra and Dharma Sutra are text books on karma yoga. Each yoga has its own scriptures.