The Invisible Book of Invisibility is a textbook about the power of invisibility. The book itself is completely invisible (hence the title), making it difficult to find and, presumably, to actually read or do anything with. Copies of the book can be found in Flourish and Blotts.
"I'm never stocking them again, never! It's been bedlam! I thought we'd seen the worst when we bought two hundred copies of the Invisible Book of Invisibility — cost a fortune, and we never found them."
The manager of Flourish and Blotts told Harry Potter in 1993 when he is buying his school books that, at some point in the past, the book store had ordered two hundred copies of the book, only to have every last one of them disappear. No one was ever able to find them, costing the store a fortune. Only the stock of The Monster Book of Monsters was considered more troublesome.
Behind the scenes
The Invisible Book of Invisibility most likely teaches invisibility spells (such as the Disillusionment Charm) as well as theory. However, it is unknown how people are supposed to read the book, considering it is completely invisible.