Until today there is no Raven Guard book from BL.It is rumored that every Legion
gets something.The actual fluff is rather short.
You may also look at Index Astartes articles or WD December 2002.
One source for Raven Guard may be http://www.lexicanum.de/
Great site with 40k info in german,english,french.
Another option: http://www.sonsofcorax.freeforums.org/
Feel free to visit and ask there