What is the Avatamsaka Sutra about? 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.


The Buddhist Religion: A Historical Introduction page 91
The Buddhist Religion: A Historical Introduction page 182
Manadala The Architecture of Enlightenment page 32
Manadala The Architecture of Enlightenment page 23 
Manadala The Architecture of Enlightenment page 135
Manadala The Architecture of Enlightenment page 135