Belgrade in your pocket is a great, essential, FREE, bi-monthly updated guide-book about Belgrade. Pick up the latest version at one of the tourist-offices or at your hotel or hostel. Or even better: download it from here. It contains everything you need to know about Belgrade, including practical maps.
We recommend the following books as a companion on your trip to Serbia:
If you are already in Serbia, the best book to buy in bookshops is Serbia in Your Hands. This excellent book is available in several languages and is also available for purchase over the internet through Amazon (see above).