I'm sure there are tons of spells that are potentially useful to a monk, but one spell really jumps out at me: ki arrow.
This spell is pretty useless to a straight sorcerer or wizard -- it lets you do your unarmed strike damage with your arrow -- but to a Monk 19 / Sorcerer 1, it might be kinda nice...
Except, a zen archer gets that ability at level 5. True it costs a swift action and a ki point but lasts 1 round.
Now, using that plus gravity bow (another level 1 spell) should mean bow damage of about 2d6 at level 5 (and you do not need the ki arrow ability with a long bow)