In Curse of Strahd, as part of the Tarokka card reading, the party are told the locations of three “treasures”; the Sunsword, the Holy Symbol of Ravenkind, and the Tome of Strahd.
Looking at the Holy Symbol of Ravenkind’s description in Appendix C (p. 222), it looks like something that a group of adventures wanting to take down a powerful vampire would want to have. The Sunsword is also an obvious thing to want when going up against a vampire; in fact, the excerpt from the Tome of Strahd (p. 252-253) describes how it’s something that Strahd fears.
But in what way does the Tome of Strahd help the party? They don’t really need to know that Strahd fears the Sunsword because any vampire is right to fear any sun blade, and we don’t need to read their diary to figure that out. The adventure at least implies that it’s as important as the other two items by including them in the same Tarokka card reading that guides the party throughout the adventure.
However, I can’t see what makes it as important as the other two items when all it does is give you a bit of history. It might be interesting for the players to learn about, but the party could quite happily skip the part where they collect that book (although they’d miss out on some background lore) and take Strahd on with only the Sunsword and the Holy Symbol of Ravenkind and still have the same odds of winning.
Having the book on their person doesn’t seem to confer any benefits to the party whatsoever. Not that every item needs to be some super-powered magic item, but I was at least expecting it to tell you a secret that would indirectly make the fight against Strahd easier or something like that. I thought the idea was that if they marched straight up to Strahd’s castle at the start of the adventure, he’d wipe the floor with them, but each of these special items they acquire along the way increases their chances of success against him, but it seems that having the Tome doesn’t affect their chances whatsoever.
The only effect it appears to have is that it makes Strahd angry, but I imagine storming his castle with the other two powerful items and trying to kill him would be enough motivation for him to be angry enough already. From the Tome of Strahd’s description in Appendix C (p. 221):
If Strahd sees, or learns from a minion, that the tome has fallen into the party’s possession, all of his other objectives (see chapter 1, “Into the Mists”) are put on hold until the book is recovered. When Strahd attacks, his preferred target is whoever has the tome.
The reason I ask is because of in-character motivation. Why would the party want this thing besides “because the card reading told us where it was, and the other two items it told us about seem pretty important so I guess this must be as well…” – I don’t want my players to find it, Sunsword in hand, and go “oh, it’s just some lore that doesn’t actually tell us anything useful at all… what was the point of that?” Note that my players aren’t all number-crunchers, they do enjoy the narrative primarily, but they still need an in-character reason to care about this Tome.
So, is there any benefit (mechanical or narrative) that the Tome of Strahd gives the party against Strahd (directly or indirectly)?