Detailed descriptions of both the refuge assembly and Guru tree can be found in many places such as Liberation in the Palm of Your Hand, Liberation in Your Hand, Joyful Path of Good Fortune, Great Treasury of Merit, and the forthcoming Manjushri's Innermost Secret from Dechenlingpress.org
Liberation life stories of lineage Masters can be read all over the place. There are also many detailed commentaries on the four classes of Tantra and the lineage Gurus of those instructions. There are also such stories written by Buton Rinpoche, Kachen Yeshe Gyaltsen, Taranatha, Janet Willis and others.
The most important point about this practice is to not only understand that the entire thing is the Guru, but that it is your actual living Guru right now in the space before you. There are many meanings, but this is the most meaningful point because it is the doorway to realization. If we always relate to images as if they aren't the living Guru in our presence, the blessings will be limited and our inner experience will not blossom very quickly.
Many Masters say it is enough to have a rough idea of the assembly and that our conviction it is a living assembly is the most crucial point. We can fill in details, but if we don't have much time for formal meditation and we get lost in the details without connecting to the most crucial point, we can waste a great deal of time. Marpa Lotsawa warned that sometimes seeking more information can itself become a distraction and obstacle to the real experience that touches our heart.
Not sayin that's how it is for you, just sayin.