All known references to the Law of Assembly 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.
The Law of Assembly appears in Discipleship in the New Age, Vol. II and nowhere else that I can find.
Quotes I found below:
This point of revelation carries the initiate to one of the highest points of contemplation; we are here concerned with his sudden apprehension—at the fourth Initiation of Renunciation—of another phase of the divine, conditioning Will. He has begun to recognise and to interpret—even though in an elementary fashion compared to what he will recognise at the ninth initiation—the destroying aspect of the Will as it expresses itself through the Law of Sacrifice. Now, for the first time (as a result of destruction), he can begin to grasp the essential building aspect of this same Will and to appreciate his future function as a creative Builder. The building here to be carried forward, I would remind you, is not the building which is distinctive of the second divine aspect—that of Love-Wisdom. It is strictly connected with that of the first aspect of Purpose, Power or Will; it deals with the processes which precede the actual creative building, the drawing up of the blueprints (if I may use such a term) “within the confines of Shamballa,” where high spiritual Beings must lay Their plans. This is a different process to the creative building process, and is related to a mysterious undertaking which is carried forward under the “Law of Assembly.”