Karnak at night has an entirely different feeling. The shows are in a different language each night, so you must adjust your schedule to get the correct one for you. We found the Sound and Light show very informative, if a little long.

Wear or bring a wrap, and don't forget the insect repellant. I would also recommend that you bring a small flashlight in case you need to leave early.

Then, you might like to return for a third visit after you visit the Valley of the Kings and try to relate the various pharaohs to the portions they built.

The Penguin Guide gives a detailed description of Karnak on pages 288-303..

The Lonely Planet Guide covers Karnak on pages 273-277.