Catalog your items. Even if you only have one or two books (or, conversely, if you have hundreds) you should create a detailed catalog of the books. Create a log of the books' titles, authors, relevant publishing dates and condition. Use digital technologies like digital photos and spreadsheets to help you keep track of your collection and refer to it easily.
Keep books out of direct sunlight. Like vampires, rare books are destroyed by direct sunlight. Natural and artificial light is fine, as long as it's not excessively bright. So, keep the books in a good level of light but out of the sun.
Check for worms, mold and other book-eaters. It only takes one contaminated book to infect your whole collection (and after the books, your clothes and furniture). Check regularly for worm trails, which are pin-sized hollowed-out spaces between the pages. You should look for mold too. If you find any of these parasites, take the affected books to an expert book-binder to have them treated.
Keep books on shelves. It's not just for the good impression that books are kept on shelves. All but the most fragile books should be stood upright on shelves, lined up next to each other with the spines facing out. Stacking books on top of each other can cause considerable damage and deformation, so keep them upright to preserve them.