Judging by the items on the book, you could only summon artifacts that are able to transform somebody into nothign at all. A bit of a ridiculous rule of the book, knowing you wanted to summon some kind of unearthly monster from where ever you thing monsters would come from, other than the underworld.
There were basically 4 artifacts for you to summon, and half of them were musical when you noticed only 2 instruments, while the others were basically just rewards. Knowing whoever created this kind of book, might want to give some people some kind of appreciation for either priceless rewards or instruments that you could play music on. Your girlfriend was too distracted by the wreckage of the castle, as she explored it, that she didn't notice you saying a spell out loud, and getting ready to summon something.