And there's the famous "Encyclopedia Galactica" in Isaac Asimov's Foundation series, it even gets its own
Wikipedia page.
I have been fooled sometimes, discovering that a referenced book is not invisible at all, just something I hadn't heard of before.
ETA: And what do you call it in something like "The World According to Garp" by John Irving, which provides parts of various books like "The Pension Grillparzer" ? Translucent? (At some more suitable hour than 2:15am I may be able to remember some of Irving's other invisible books.)