I just checked and as far as I know the book gives you a permanent Speak with the dead if you pass all the skill checks, which I never did because with my luck at RNG there is simply no way I can pass three consecutive checks. I always give the book to Astarion who does indeed now have the ability to speak with the dead and if it doesn't come from the book and don't know where it's from since no other piece of equipment he's wearing grants that skill.
