The phrase "True Will" does not appear in the Book of the Law. Nevertheless, Crowley's various commentaries on the Book routinely postulate that each individual has a unique and incommensurable True Will that determines their proper course in life. This invention of Crowley's appears to be an attempt to explain how some actions may be wrong (or "false") when "There is no law beyond Do what thou wilt." (CCXX III:60) Actions that conform to True Will are thus considered to be correct, while willed actions that deviate from True Will may nevertheless be wrong.
In general, it is supposed by contemporary Thelemites that no one can know the True Will of another, but in Crowley's essay "The Secret Conference" (written under the pseudonym of Gerald Aumont, and prefaced to The Heart of the Master), he suggests that a technique may (indeed, must) be devised, by which a child's True Will may be discovered at birth, or as early as possible in life, in order to permit the correct ordering of society. We can speculate that Crowley had an astrological method in mind, although subsequent historical developments may lead us to consider genomics as a better candidate for the hypothetical technique.
In Crowley's ethical treatise "Duty", he identifies True Will with the Nature of the individual. This capitalized "Nature" may be compared with the "Perfect Nature" of earlier Gnostic systems, which was another term for the personal daimon or augoeides, usually referenced by Crowley as the Holy Guardian Angel. (For this use of the term "Perfect Nature," see Corbin's Man of Light in Iranian Sufism.)
"The Message of the Master Therion" (Liber II) is a seminal document that attempts to delineate the doctrine of True Will. By reference to "Liber Thisharb", Liber II implies a theory of metempsychosis, whereby the individual True Will is the resultant of a person's prior incarnations.
In "De Lege Libellum" (Liber CL), Crowley defines True Will as the will which does not "rest content with things partial and transitory, but ... proceed[s] firmly to the End," and in the same passage he identifies that "End" as the destruction of oneself in Love.
Quotations from Aleister Crowley
"The most common cause of failure in life is ignorance of one's own True Will, or of the means by which to fulfill that Will." (from Magick, Book 4, p. 127)
"A man who is doing his True Will has the inertia of the Universe to assist him." (from Magick, Book 4, p.128)
"One can not do one's True Will intelligently unless one knows what it is." (from Magick, Book 4, p.174)
"Know firmly, O my Son, that the True Will connot err; for it is thine appointed Course in Heaven, in whose Order is Perfection." (from Liber Aleph, p. 13)
"True Will should spring, a fountain of Light, from within, and flow unchecked, seething with Love, into the Ocean of Life." (from Little Essays Towards Truth, p.76)