Sri Aurobindo
Letters on Yoga
2. Integral Yoga and Other Paths
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The ascension and descent of the Force in this yoga accomplishes itself in its own way without any necessary reproduction of the details laid down in the Tantric books
. Many become conscious of the centres, but others simply feel the ascent or descent in a general way or from level to level rather than from centre to centre, that is, they feel the
Force descending first to the head, then to the heart, then to the navel and still below. It is not at all necessary to become aware of the deities in the centres 
according to the Tantric description, but some feel the Mother in the different centres. In these things our sadhana does not cleave to the knowledge given in the books, but only keeps to the central truth behind and realises it independently without any subjection to the old forms and symbols. The centres themselves have a different interpretation here from that given in the books of the Tantriks.
Current publication:
Sri Aurobindo. Letters on Yoga // SABCL.- Volume 22. (≈ 28 vol. of CWSA).- Pondicherry: Sri Aurobindo Ashram, 1971.- 502 p.
Other publications:
[A letter: ] Sri Aurobindo. Sri Aurobindo and Mother to Prithwi Singh: Correspondence (1933-1967).- Mysore: Mira Aditi, 1998.- 183 p.
Sri Aurobindo. Letters on Yoga. II // CWSA.- Volume 29. (≈ 22-24 vol. of SABCL).- Pondicherry: Sri Aurobindo Ashram, 2013.- 522 p.
Sri Aurobindo. Letters of Sri Aurobindo: In 4 Series.- Second Series [On Yoga].- Bombay: Sri Aurobindo Sircle, 1949.- 599 p.