I apologize if this has already been addressed, but I ran into a game breaking quest bug today. I had already completed the quest for Septimus Signus where you get the book that increases all skills in one style branch (ie melee, stealth, magic). Upon talking to Paarthurnaux, I went to the College of Winterhold in order to talk to the orc. When I talk to him, there is no option to progress the Elder Knowledge quest. Again, my apologies if this has been addressed in this forum.

Read the two books he put on his desk.

I only see The Rise and Fall of the Blades, and when I read it, it doesn't give me a prompt for anything.

So I just tracked down the two books you're supposed to be given (I started another character and got to this quest line). After reading them, nothing happened. So it's definitely a bug/glitch.

So disregard this topic. In a fit of stupidity, I forgot to actually grab the elder scroll when doing the mission for Sigmus. Problem solved.