Not only can you decorate your leather item, leather itself can be the decoration. Book binding is a great example, which has recently enjoyed renewed popularity. The way you look at this project depends on the condition of the book.
You may need to repair the spine and actual page bindings in order to install a new cover. In any case, this first step would be to remove the old cover. http://www.philobiblon.com is a site that has very detailed instructions to walk you through this. Half of the fun of leather book binding is learning about it. http://palimpsest.stanford.edu/don/dt/dt0412.html has information about book binding past, present, and future.
Their information is accurate and interesting! You can start from scratch with a hide, or order leather book binding supplies from a reputable supplier such as http://www.hewit.com
There is more to book binding than slapping a cover on. If you want a rich, professional appearance, you'll find numerous books at your local library or in-depth information on the Internet on the process of book binding. Hand-bound books are considered rare, unusual, and in some cases priceless. Therefore, it would be worth your time and money to do it right and enjoy the results. (or profits if you are doing it professionally)