There is only one known reference to the Law of Ascension by Alice A. Bailey (AAB) in her voluminous writings and those given to her by Djwhal Khul (DK). Links on these pages of The Laws of Alice A. Bailey are to the online books at the Lucis Trust website.
Although the law is mentioned specifically only once, there are many references to “Ascension,” for example:
A Master learns the meaning of each confining form; then He assumes control and wields the law upon the plane consistent with the form. He has then outgrown the form and discards it for other and higher forms. Thus, He has progressed always by means of the sacrifice and death of the form. Always, it is recognised as imprisoning, always it must be sacrificed and die so that the life within may speed ever on and up. The path of resurrection presupposes  crucifixion and death, and then leads to the mount whence Ascension may be made.
The Resurrection and Ascension, the final triumph which enables the initiate to sing and to know the meaning of the words: “Oh death, where is thy sting? Oh grave, where is thy victory?” [xxiii] 21
What is a law?
… a law is but the spiritual impulse, incentive and life manifestation of that Being in which he lives and moves. A Treatise on White Magic , INTRODUCTORY REMARKS
A law is an expression or manifestation of force applied, under the power of thought, by a thinker or group of thinkers. Discipleship in the New Age, Vol. I , SECTION ONE – DISCIPLESHIP IN THE NEW AGE – Part 2
… a law is in reality the effect of the life of a greater entity as it encloses a lesser within its living processes.
Esoteric Healing , Part Three – The Fundamental Laws of Healing
… a law is but the effect of the continued intelligent activity of the Life aspect as it works in conjunction with matter. Esoteric Psychology – Volume I , 5. The Rays in Cyclic Manifestation
A law is only an expression or manifestation of force, applied under the power of thought by a thinker or group of thinkers. Esoteric Psychology – Volume II , II. The Seven Laws of Soul or Group Life – Part 5
A law presupposes a superior being who, gifted with purpose, and aided by intelligence, is so coordinating his forces that a plan is being sequentially and steadily matured. A Treatise on White Magic
Also interesting in the single quote below on the Law of Ascension is the comment on the “I AM” movement.
Quotes I found below:
As regards the Hierarchy itself, speaking esoterically and technically, its Members (many of Them) are “being abstracted from the middle point of holiness and absorbed into the Council of the Lord.” In other words, They are passing onward into higher work and are becoming custodians of the energy of the divine will and not simply the custodians of the energy of love. They will work henceforth as power-units, and not just as units of light. Their work  becomes dynamic instead of being attractive and magnetic, and is concerned with the life aspect and not just with the soul or consciousness aspect. Their places are being taken—under the Law of Ascension—by Their senior disciples, the initiates in Their Ashrams, and (under the same great process) the place of these initiates, who are thus being “raised” to more important work, is being taken by disciples and probationers. It is this truth, misinterpreted and shockingly travestied, which lies behind the teaching anent the so-called Ascended Masters, put out by the leaders of the “I AM” movement, thus prostituting and bringing down almost into the realm of cheap comedy one of the most notable happenings which has ever taken place upon our planet.
You are welcome to post your comments and understanding of this law.