I consider to be the "big 3" that NHTSA pulls out:
Motorcycle Helmet Effectiveness Revisited (from March 2004):
Calculating Lives Saved by Motorcycle Helmets (from August 2005):
An Analysis of Motorcycle Helmet UseIn Fatal Crashes (from August 2008):
Note that they often point to the FARS data (http://www-fars.nhtsa.dot.gov)
which regularly only records whether a Helmet was worn, not whether it
contributed as a factor.