Can you mount a gas fireplace in a bookcase?

It depends on if it is a self-contained unit or not. For the most part, no, because the unit needs to have a certain amount of clearance between the four sides and its surroundings. Some have to be vented. You really need to check your owners manual to get the specifications.

Edit : Look for "clearance to combustibles" There are ways to alter the clearance to combustibles in most places using metal sheeting, bricks, etc.