A spell is cast by chanting a number of syllables. These syllables are so cogent that they must be forced into the brain by a trained mind. Once the sorcerer has cast the spell, he is no longer able to do so until he again consults his books. This indicates to me that the syllables no longer reside inside his brain, preventing him from casting it again.
I don't remember the "forget" part from either of the first two novels. Is it in there?