This one's kind of fun.

In Golden Axe 2, if you go to the Options screen and set the type of magic to Special

Then play the game normally until you come to a boss. When the music changes, hold in the A button (or whatever button you have designated for Magic

Don't let it go until the battle's over and the screen transitions to black. Then, when you go to the bonus stage, don't press any buttons at all.

It'll kind of look like the bonus stage will go on forever until you thwack the enemies walking around, but it will eventually end.

When you start the next stage, your character will immediately unleash a magical attack

After which, you'll see that your stockpile of magic books has suddenly grown enormous!

If you use more than your character's normal maximum, you can occasionally use other characters' spells

But, if you use too many, the game will freeze, so staying within your characters' limits is probably for the best.