Spellsinger by Alan Dean Foster? Did it have “critters” in it?
Not Spellsinger.
I remember these things:
His first “spell” was part of “A Visit from Saint Nicholas” where he teleports a bunch of bad guys up to a distant wall by quoting the poem.
There is a magician who is helping him who cringes everytime he makes up a spell by creating poetry, because of the intrinsic dangers of just making stuff up.
He gets a magic sword and, through poetry, gives it a mono-filament edge.