MathematicalPhysicist said:
Is there some review article which covers the most important literature?
...
Rovelli's 2008 review article
http://relativity.livingreviews.org/Articles/lrr-2008-5/
You point to Don Marolf's 2003 graded catalog of pedagogical resources, including websites, popular articles, books, and professional articles classified by level (introductory, advanced,...). That's primo, almost defines a new kind of review article. It lists 120 sources including much that is online. I don't happen to know anything recent (like 2008-2010) that is comparable.
Rovelli's 2008 Living Reviews article is online, has 300 source articles, has a lot of URL links, and gives an overview as of May 2008. It is however organized differently, more like the traditional scholarly survey.
If you find other useful stuff, please post it! Particularly if it is recent (post-2007).
I keep a bunch of QG links handy here:
https://www.physicsforums.com/showthread.php?p=2595126#post2595126
Not complete in any sense, or especially focused.
Since you mentioned Don Marolf's 2003 article, you might be interested to know that he is organizing the July 2010 GR19 conference---where there will be parallel sessions on LQG and on String. There is a 2 week QG school just preceding the GR19 which Marolf was instrumental in funding and setting up, designed for advanced grad students and others who want to get into QG research.
marcus said:
23 June-3 July 2010, Americas QG school at Morelia:
http://www.physics.ucsb.edu/~pasi/
5-9 July 2010, GR19 conference in Mexico City
http://www.gr19.com/index.php
(LQG will have a session of GR19 which will serve as this year's "Loops 2010"