The polycarbonate I suggested would be a good start. However, all light will fade colours eventually, including the visible. Either make doors infront of your bookshelves or leave them out of the direct sunlight. Polycarbonate is your best bet if you wish to display them. I'm not sure where you might buy it, B&Q, a builder's merchant or a garden centre might be a good start. Be sure that you are being sold polycarbonate though and not some other plastic, as in my experience non-chemists use terms like polycarbonate, perspex or whatever imprecisely.