Where can I buy the Invisibility spell book?
From what I've experienced, the spell sellers in The College of Winterhold will in fact begin marketing the spells to you 10 degrees prior to you can acquire the perk related to that certain effectiveness degree. I saw this when I was educating Alteration: Tolfdir marketed me Telekinesis when I just had a change ability of 40. I additionally had a comparable experience with my Conjuration tomes. So you need to have the ability to acquire that tome as soon as your Illusion gets to 65 and afterwards just from the Illusion instructor at the university.
I do not see why this would not collaborate with various other colleges of magic too yet have not had an opportunity to examine it myself yet. If any person else can validate, it would certainly behave.