Guest book 
Can you wait and buy it when you arrive? NO
Long plane trips, endless waiting at bus parks, or simply sitting down and relaxing - they all lend themselves to a good book. However, books in Uganda are expensive, hard to find and there is limited choice even in the larger book shops, so choose one or two books before you leave home, and ideally pick ones you don't mind leaving behind at the end of your trip.
Paperbacks are often cheaper and lighter than hard backs, and are easier to shove into small spaces in a bag, so they should be your preferred format. Its a backpacking tradition to abandon books you have finished, leaving them on the shelves of your hostel for others to enjoy after you've gone. Some places allow you to take another book from the shelves and carry it with you until that too is finished, and leave it at another hostel, so there must be some very well traveled books out there.
Most books you find this way will be fiction- often thrillers and romances - so if you have a passion for something more specialised, don't expect much luck renewing your traveling library en route.
At the end of your trip, offer your remaining books to the hostel where you are staying, a friendly Ugandan who may value them more that you'd believe, or a local school where they are bound to be put to good use. And the space you save, makes room for souvenirs to take home.