The single most important thing not to lose in 1990.

You popped it in front of a cartridge and when you started the NES it would first bring you to a code screen where you could enter your magic genie code and start the game. It worked by modifying memory in the game much like an old school game 'trainer' would do. (hex editing memory is an art of its own). So the book would give you these codes that would do things like give you unlimited ammo, lives, skip levels, and so on. You could also try your luck at semi-random or even completely random codes and see what happens. Usually it would result in broken fuckery, but if you tried to combo up codes there were very interesting and unexpected results (new colors schemes, new 'levels', weird shit altogether. Cool toy)