While the primary purpose of a bookcase is to store your books, and therefore the majority of the item will consist of shelves, it's also good to know that you can buy bookcases with other storage too. Many people store various things in bookcases, since many of them come with drawers or small cupboards too. Since you might be purchasing a large item of furniture, it's definitely good to think about any future plans you may have for your home, particularly if you have young children and they are likely to need more storage space as they grow older and progress in school.
Aside from storing your favorite (and not so favorite) books, you can also use a teak bookcase as a form of display cabinet. If you really like a large bookcase but you know you don't have enough books to fit it, consider displaying your ornaments and other items on the empty shelves. Aside from being a useful storage area for beautiful items you want to show off to people, it also serves the purpose of 'saving' the space for when you may need further storage for books in the future.