The Pagemaster

Level skip:
Pause game play and press Left, Up, Left, Up, Start, B, Right, A. Alternatively, press Up, Down, Left, Right, B, A, Select.

Level select:
Press B, A, X, Select, Start during game play.

Alternately, press B, A, L, Down, Y, B, A, L, Down, Y at the title screen. Then, pause game play and press Select to advance to the next level.

Final level:
Enter [Eyeball], E, E, [Key], [Blob], B as a password.