Thin booklets or pamphlets can get squashed and crushed if you try and store them on your bookshelves with your regular books. There are a number of different options for storing such material, but if you like to keep things all together then try placing them inside a large Ziploc bag with a stiff archival support board behind them. The support board can be made of acid-free cardboard or an inert plastic like polypropylene (Corflute and Corex are two corrugated varieties available). You can use other types of plastic sleeve too (stick to polypropylene or polyethylene), but the Ziploc bit helps keep dust out. This gives them enough rigidity to hold their own against the big books.