» Sun May 13, 2012 10:31 pm
I don't think this is the case. She sells you the tome of a Novive spell, meaning anyone can cast it. You have to cast it on the rune symbol where you should be standing. If you're not standing there, go and search for it along the path to the castle. If this doesn't work, fast-travel somewhere then return, and/or reload saves and try again. Could be a bug, but I've not read anything about it.
It's definitely not mana because they max' any spell she'll give you will cost 34 mana.
PS: People should stop trying to help unless they have knowledge of the problem. I've had a huge bug on a Whiterun quest and been asking for help, I've had one guy tells me it's a bug... When 'bug' is in the topic title. Jus' saying.