"Red notebook!" is an exclamatory approximately equal to the meaning, "that is so obviously obvious and apparent that you must be an absolute moron not to have caught it."
In other words, DUH. Red notebook!
Usually accompanied by rolling eyes or, in text or IM conversations, an eye-rolling emote.
"Omg you mean all that music I downloaded from Pirate Bay was pirated?!"
"Duh! Red notebook!"
"Hey, where's Allie?"
"I'm right here. Red notebook!"