Where can I buy the Invisibility spell book?

You can only buy it from the illusion trainer in the College of Winterhold, but he won't sell it until your illusion is of sufficient expertise (i.e. expert level).

From what I've experienced, the spell merchants in The College of Winterhold will actually begin selling the spells to you 10 levels before you can buy the perk associated with that particular proficiency level. I saw this when I was training Alteration: Tolfdir sold me Telekinesis when I only had an alteration skill of 40. I also had a similar experience with my Conjuration tomes. So you should be able to purchase that tome once your Illusion reaches 65 and then only from the Illusion trainer at the college.

I don't see why this wouldn't work with other schools of magic as well but haven't had a chance to test it myself yet. If anyone else could confirm, it would be nice.