I read enough on that site to make it quite clear that the Great Work is the actual book they seem to worship.
Regardless, this still seems rather religious, even if some sects are less so. By reinterpreting "great work" it sounds like so many rationalizations of biblical belief.
From: http://en.wikipedia.org/wiki/Great_Work
"The Great Work is, before all things, the creation of man by himself, that is to say, the full and entire conquest of his faculties and his future;"
How you do it doesn't matter. Thelema uses a lot of fancy language because it's Edwardian and its founder was a poet.
It's best understood in historical context. The purpose of Thelema was to free post-Victorian society from the social and moral superstitions and fixations which prevented people from really being themselves. It was essentially the first stirrings of the movement which found full expression sixty years later in the "New Age". Crowley referred to it as a "New Aeon"; poetry strikes again.
The essential tenet of freeing oneself from mistaken ideas, choosing a life goal and doing it, is as valuable today as it was then, but you don't have to call yourself a Thelemite to do it. To a great extent, that "meme" is now mainstream.
perhaps a place to start: https://en.wikipedia.org/wiki/Hermetica