Brain Drain

Brain Drain is the twelfth and final book in the Nintendo Adventure Books series.

Plot summary
The story from the back cover:

"Yoshi's Head Aches Evil Iggy Koopa has created a device to switch the brains of the creatures in the Mushroom Worlds. When he gives Yoshi the brain of a sinister Koopaling, Mario and Luigi jump into action. With the help of Dr. Sigmund Shrinkasaurus, they try to stop Iggy before all of their friends in Dinosaur Land lose their heads! As in all Nintendo Adventure Books, you control the action. Only you can make the choices that allow Mario and Luigi to halt the chaos caused by Iggy Koopa's brain twister machine. Help them crack codes, solve puzzles, conquer mazes, and subdue enemies. You and the Super Mario Bros. must outwit a whole new generation of Koopa creeps&mdash;or go down in final defeat."

Items
An unidentified piece of paper is shown as well, but it is never collected. Presumably, it is the "Plumber's Oath" that Mario has Luigi memorize so they can identify each other no matter what bodies they are in.
 * Pizza
 * Wrench
 * Tape
 * Fungus Up
 * Morel Moxie
 * Lichen Cola

Characters

 * Bowser (mentioned)
 * Bowser's Cousin
 * Doctor Sigmund Shrinkasaurus
 * Iggy Koopa
 * Luigi
 * Mario
 * Morton Koopa Jr.
 * Mushroom King
 * Princess Toadstool
 * Toad
 * Yoshi
 * Wooster
 * An unidentified Koopaling (possibly Morton) also switches minds with Yoshi at the beginning of the book.

Score chart

 * Over 800 pts. – Tidal Brain Waves
 * 600 to 800 pts. – Super Stream-of-Consciousness
 * 400 to 599 pts. – Sewer Smarts
 * 200 to 399 pts. – Brain Needs a Wash
 * 0 to 199 pts. – Spaghetti Synapse