Be careful: The standard Fortress Europe book covers late-war generally. The will include winter-specific rules like "Deep Snow" (which halves movement for wheeled vehicles) and "Whitewash" (camouflage painting giving an Ambush bonus).
Battlefront Miniatures (the makers of Flames of War) has shifted toward a "Living Rulebook" model. However, the primary source for the Bulge is the supplement book (usually Version 4 or V4). Flames Of War Bulge German Pdf