DevilutionX says in the manual that you can assign spells to quick buttons using F5-F8. That doesn't seem to do anything for me. I've tried pressing it with it as the selected spell, I've tried pressing it from the spellbook screen, but it never seems to work. I don't know what I'm missing.

I've gotten to level 13 of the labyrinth and I'm at the point where rapid switching of spells is getting close to vital for survival.

Diablo only has F5-F8, but DevilutionX has 12 quick spells and you can map them to any key.

To assign a spell to a button you open the quick book (press S or click the Spell Icon in the lower right of the main panel).
Then highlight a spell with the cursor and then press the quick button you would like to assign it to.
You should now see the button name overlayed on top of the icon, and can press the it to switch to that spell in game at a later point.

On a side note you can also switch on quick cast in DevilutionX, this will change the behavior so that instead of switching to the spell you will instantly cast the given spell when pressing on the assigned quick spell button.

Thanks. I don't know why I couldn't find that.