fromEnglish Grammar Today
Beside
Besideis apreposition. It means ‘at the side of’ or ‘next to’. It is rather formal:
He would like to take a photograph of us. Would you come and sitbesideme?
There was a small tablebesidethe bed, on which there was a book.
Besides
Besidesis apreposition or a linking adverb.It means ‘in addition to’ or ‘also’:
What other types of music do you likebesidesclassical?
As a linking adverb, we usually put a comma before and afterbesidesin writing:
I don’t think going for a walk is a good idea. It’s quite cold, and,besides, it’s getting late and we don’t want to be out in the dark.