Whoops. Seems I just had to purchase the Lich Lord expansion for Frostgrave.
It's the same size as the main book but this time a 64 page perfect bound, soft cover publication. It's a great read with some excellent scenarios and superb additions to the soldier list. Because it's primarily a scenario book it included more background fluff to read, which I enjoyed – each scenario building up to an inevitable finale. Then there's new spells and the opportunity to take your wizard to the 'Dark Side'. Now we're talking!
Obviously I haven't played a single game of Frostgrave, but as a casual observer it certainly seems like a worth addition. Even if you never intend to play Frostgrave there are some interesting scenario ideas that could be adapted for other games.