Which is the right tense for talking about past activities that are true in the present as well [closed]
In basic English, both of them are correct, nevertheless, when you say the book's name was... you're expressing that the name was Death by Black Hole and may, but doesn't have to be anymore - Either way, you say that the book's name was it at the point when you were reading it.
However, this is a tiny detail, which may even be unnoticeable in spoken English.