In reputable usage, beside is usually a preposition meaning "by the side of." Besides, also a preposition, meaning "except," is more commonly used as an adverb in the sense of "more over" and "in addition to." "I am resting beside (not besides) the stream." "I have more work to do; besides (not beside) I am not in the mood to go."
Dictionary of problem words and expressions. Harry Shaw. 1975.