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Quoted from Better Homes and Gardens: Max out available space in an office closet with adjustable wire shelving. Use open baskets for supplies you access regularly and boxes with lids for infrequently used items. Divide papers to be filed in layered trays and set up file folders for items that require action in the upcoming month.
office in a closet... we had an office in a closet once. It didn't look quite like this though ; )