I am in Helm's Hold, and at the Altar of Helm - I try to put the book on the Altar, but I get a message saying that if I had the proper book, I could use it to activate the Altar. How do I put the book on it? I clicked on the Altar but am getting that message. Can anyone help me?

You need two books.
The Black Grimoire (to banish the demon) is on a bookshelf in one of the rooms.
The Book of Helm (to summon the Guardian Spirit) you get off one of the false Helmites. (probably the book you have)

Thanks so much!