Sri Aurobindo
Letters of Sri Aurobindo
Volume 2. 1935
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1935
Your above answer regarding the spiritual planes and supramental planes urges me to inquire about the exact difference between spiritualisation and supramentalisation.
Spiritualisation means the descent of the higher peace, force, light, knowledge, purity, Ananda etc. which belong to any of the higher planes from higher mind to Overmind, for in any of these the Self can be realised. It brings about a subjective transformation; the instrumental Nature is only so far transformed that it becomes an instrument for the Cosmic Divine to get some work done while
the self within remains calm and free and united to
the Divine. But this is an incomplete individual transformation – the full transformation of the instrumental Nature can only come when the Supramental change takes place. Till then the nature 
remains full of many imperfections, but the Self in the higher planes does not mind them, as it is itself free and unaffected. The inner being down to the inner physical can also become free and unaffected. The Overmind itself is
subject to limitations in the working of the effective Knowledge, limitations in the working of the Power, subject
to a partial and limited Truth etc. It is only in the Supermind that the full Truth-Consciousness comes into being.
Current publication:
[Largest or earliest found passage: ] Doshi, Nagin. Guidance from Sri Aurobindo: Letters to a Young Disciple.- In 3 volumes.- Volume 1. 1933-34
Other publications:
Sri Aurobindo. Letters on Yoga // SABCL.- Volume 22. (≈ 28 vol. of CWSA).- Pondicherry: Sri Aurobindo Ashram, 1971.- 502 p.
Sri Aurobindo. Letters on Yoga. II // CWSA.- Volume 29. (≈ 22-24 vol. of SABCL).- Pondicherry: Sri Aurobindo Ashram, 2013.- 522 p.
Doshi, Nagin. Guidance from Sri Aurobindo: Letters to a Young Disciple.- In 3 volumes.- Volume 1. 1933-34
Sri Aurobindo. Letters of Sri Aurobindo: In 4 Series.- Forth Series [On Yoga].- Bombay: Sri Aurobindo Sircle, 1951.- 652 p.