I'm not really expecting a short summary, but I'd be happy to get one. This is a 1600 page sutra in English (and cost over $100 on Amazon). It gets reference a lot in the Brahma Net Sutra and Huayan Buddhism.
Given that, what is the best way to dig into this... is there a Cliff Notes version?
And what is the Avatamsaka Sutra prescribing its readers to do?
During the beginning of the 4th century CE, Buddhist scholars began composing a new series of Sūtras with the purpose of synthesizing and organizing concepts from earlier Mahāyāna Sūtras, a task which the authors viewed as the third turning of the wheel of Dharma. Three Sūtras from this period left a major impact on the intellectual endeavor of Mahāyāna monastics: the Avatamsaka Sūtra, the Lankāvatāra Sūtra, and the Sandhinirmocana Sūtra. The underline theme found in these three Sūtras, including the Avatamsaka Sūtra is the experience of the universe formed by consciousness detailed within epic narratives.1
The Avatamsaka Sūtra is also know for expounding the teaching about the ever-abiding Buddha-nature that constitutes ultimate reality.2 However, some scholars believe the Avatamska Sūtra was composed earlier between 200 BC and 200 CE and played a major formative role in the development of Buddhist thought.3 Nonetheless the Sūtra played an important role in the formation of Mahāyāna Buddhist teachings, which cannot be down played.
"The Avatamsaka Sūtra or Flower Garland Sūtra was translated into Chinese by Buddhabhadra between 408 and 410 CE. It gives a very detailed account of Shakyamuni/Vairochana’s quest for enlightenment, and offers the view that existence is the combination of individual identity and interdependence, a crucial theme in the development of Indian Buddhist thought. The Avatamsaka Sūtra became influential in the late sixth and seventh centuries through the works of monks such as Fashun (557-649 CE) and Zhiyen (602-668 CE), and later was the central text of the semi-esoteric Huayen order in China, which is known as Kegon in Japan.”4
The mandala principle found in the Avatamasak Sūtra, “conveys the most extraordinary visions that engage the audience in creating a setting and developing a receptivity that makes it possible for them to receive the Universal Vehicle teaching of the profound freedom and magnificent destiny of all living beings.” 5
An example of this found in the Avatamsaka Sūtra, "the Buddha is surrounded by an enormous host of visitors, many from other universes, who sit in flower towers thousands of miles high, flown at more than warp speed across the reaches of space from worlds beyond as many worlds as there are grains of sand in sixty-two Ganges riverbeds. The towers are arranged in the cardinal directions, intermediate quarters, and at the zenith and nadir in a classical mandala formation. During the teaching, Shakyamuni Buddha now and then transforms himself into the sapphire-blue Vairochana, radiating magic light rays from his forehead that temporarily bestow on each member of the audience the intensely accelerated vision of all their past life experiences and all their future life attainments, up to and including attainment of perfect buddhahood in a future universe and performance of buddha deeds for the sake of all beings. At the end of the teaching, the Bodhisattva Maitreya introduces the hero of the tale, the pilgrim-seeker Sudhana, a banker’s son, into the magical tower of Vairochana. There Sudhana beholds the entire evolutionary history of Maitreya from a self-centered, unenlightened being to a functioning buddha.
At the same time, the panorama unfolds in infinite resonant variations in every atom of the universe, each containing infinite micro-universes, which contain infinite worlds where infinite living Sudhanas enter infinite Vairochana towers and behold infinite evolutionary panoramas of infinite living Maitreyas.”6
The Avatamasak Sūtra utilizes a narrative mandala principle for the reader, in which Buddha transports his audience into a new dimension of consciousness exploring the potentialities of their possible destiny.
According to a page on About.com ...
"The Avatamsaka Sutra is a Mahayana Buddhist scripture that reveals how reality appears to an enlightened being. It is best known for its sumptuous descriptions of the inter-existence of all phenomena."
The content: http://www.cttbusa.org/fas1/fas_contents.asp
The cliff note version (?): http://www.buddhism.org/Sutras/2/Avatamsaka_Sutra.htm
Not sure this is what you are looking for though. Good luck.