This book is perhaps the best general overview of its chosen subject area. One part I enjoyed were the discussions of how much the Balkans once had numerous transport hubs for Europe, Belgrade being one but not the only example:
Thessaloniki was among the cities that experienced an economic boom. The city was home to the third most important port in the Ottoman Empire. Between 1880 and 1912, the volume of goods traded in Thessaloniki doubled from one to two million tons. There were railway connections to Vienna and Istanbul. new local factories produced flannel, woolen, and cotton products, as well as cigarettes. Important exports included leather, silkworms, raw materials for textiles, and especially tobacco, the production of which took off around the turn of the century. Thirty-eight of fifty large companies in the city were owned by Jewish families…The majority of these families specialized in the import-export business.
Between 1850 and 1913, the value of exports from Serbia increased by a factor of five, and from Romania by a factor of fourteen.
You can order the book here. I think about the Balkans a great deal (and enjoy visiting there), if only because they are one simple alternate scenario for what the rest of world history will look like.