Tamodan Varison
Grave Guard
*)But the point is you don't ALWAYS want that
if my skeletons are fighting something, there's no point in reducing their Toughness, 'cause I won't kill them anyway. If my Ghouls are fighting something with an armour save, I want the 6th spell. But it's fairly useless for my Grave Guard of Blood Knights. Against most Elves, Daemons and Warriors, -D3 WS won't make a difference.
*)All those are HELPFUL. As I said, -3 LD is useful against Daemons. But it won't change the flow of the combat. And even if it does, I might not wanna risk sacrificing 3 PD for it. It can be useful, but it doesn't have to be. So sometimes, e.g. when 2/6 spells are spells I don't want to cast normally. So perhaps I use it once, maybe twice a battle. But then I paid 20 points per attempt.
*)That's too much, as I have access to 5/6 spells without having to buy the Cursed book. And that gives me the flexibility to choose which spell I cast.
*)And when using 2 dice with a lvl 4, the power level will be higher 4/6 times than the Book will cast it on, on average.
So summarizing, it might be useful, particularly if you use a list low on magic. But in my opinion, that's not an optimal VC list
but there's stuff way higher on my arcan item list I need more than this rather frivolous item 
*)All those are HELPFUL. As I said, -3 LD is useful against Daemons. But it won't change the flow of the combat. And even if it does, I might not wanna risk sacrificing 3 PD for it. It can be useful, but it doesn't have to be. So sometimes, e.g. when 2/6 spells are spells I don't want to cast normally. So perhaps I use it once, maybe twice a battle. But then I paid 20 points per attempt.
*)That's too much, as I have access to 5/6 spells without having to buy the Cursed book. And that gives me the flexibility to choose which spell I cast.
*)And when using 2 dice with a lvl 4, the power level will be higher 4/6 times than the Book will cast it on, on average.
So summarizing, it might be useful, particularly if you use a list low on magic. But in my opinion, that's not an optimal VC list