I had never heard of this before, but I've been reading a book about John Cage and his experiences with zen, and it's mentioned in a section about D. T. Suzuki, who lectured about it in New York in the late 1950s. This seems like quite a complex sutra; the only English translation is over 1,600 pages long, and it seems very florid in style, from what I've seen (looking at Amazon previews).
Here's a quote from Suzuki:
"As to the Avatamsaka-Sutra, it is really the consummation of Buddhist thought, Buddhist sentiment, and Buddhist experience. To my mind, no religious literature in the world can ever approach the grandeur of conception, the depth of feeling, and the gigantic scale of composition, as attained by the sutra. Here not only deeply speculative minds find satisfaction, but humble spirits and heavily oppressed hearts, too, will have their burdens lightened. Abstract truths are so concretely, so symbolically represented here that one will finally come to a realization of the truth that even in a particle of dust the whole universe is seen reflected—not this visible universe only, but a vast system of universes, conceivable by the highest minds only."—D. T. Suzuki
How does this sutra fit in with the Zen of Dogen?