Out of curiousity, how is the Tome of Strahd supposed to help the PC's? It's great that they found the truth behind Strahd's sordid past, but I don't know how that helps...
It's up to the DM to incorporate understanding Strahd's personality into the story. Otherwise, the book's only listed power is to piss Strahd off and tag whoever is carrying it as his favored target.
As earlier, in my campaign, understanding Strahd's obsession was key to undoing him. This meant, in some form or another, taking Ireena from him, and doing so in such a way that his rage would be instant so that he would abandon his "invincible tactic" (of running through walls, healing up, then repeat).