Where is it???? ive looked everywhere. i chose vampire side BTW

Read the book and it should give a quest for "investigate the Arkngthamz Ruins"

But Where is the book located?

Just check on various bookshelves, I found it in a vanilla dungeon. I think it's randomly placed.

When you went to the dawnguard castle, it was on a bookshelf. It is a white book. I do think there are others around but I do not know where.

Thanks Devine