http://blogs.scientificamerican.com/information-culture/2012/05/14/wikipedia-journal-articles/
If there's an "encyclopedia" of math education, it's NCTM's "Second Handbook of Research on Mathematics Teaching and Learning," a 1200+ page book that will cost you at least $200. The first handbook was published in 1992 and the second in 2007. Will we have to wait 15 years for a third edition? If I had my way, the handbook would turn into a wiki, with pages maintained by experts and revisions suggested by researchers as new knowledge enters the field.