What? No! They get added to Shalidor's Library or Eidetic Memory. Don't you want to fill that up?
Seeing as it's possible to join the Mages Guild "later in life", and you start with the level that you have from the Lorebooks you've already read, I would be very surprised if a person would lose out on Mages Guild levels if they later decide to increase it past 10 and you've already read all the Lorebooks. It's not like other Achievements which have been added later, where the numbers weren't counted before.
Hmm, I've heard about eidetic memory but am not sure what it is, is it useful for gameplay? Same about shalidor's library, what is it, just an achievement?