Prior to and during the 1911-13 Balkan Wars, the largest military in the Balkans, after Austria-Hungary's, was Bulgaria's. After the wars, it was Serbia.

Bulgaria had a substantial military, access to the Aegean AND the Black Sea, and it was also well placed: If Bulgaria joined the Entente faction (the 'Allies'), which it did not, it could threaten Turkish control over the straits, and secure the borders of Serbia and Romania. If they joined the Triple Alliance faction (the 'Central Powers), which they DID, they could secure the Ottoman position vis-a-vis the Straits, and threaten the Serbs AND the Romanians. As well, Tsar 'Foxy' Ferdinand I was German, and the Bulgarians were spoiling for a rematch after their humiliation in the Second Balkan War.

When the Bulgarians entered the war in 1915, it spelt the death knell for the Serbs and Montenegrin, who were low on supplies, weak from the fighting in 1914, and now had to contend with Austro-Hungarian and German troops to the north, and Bulgarian Armies poised along the entire eastern border of Serbia; the resulting campaign was bloody and fiercely fought, but ended in the occupation of Serbia, Montenegro and Albania. From then on, it was a matter of keeping the Anglo-French and Serbian-Montenegrin forces holed up inside the perimeter of Salonika, "the largest POW camp on the continent".

That was, until 1916, when the Romanians, optimistic after the Russian successes in the Brusilov Offensive, entered the war on the side of the Entente. After the initial Romanian Offensive, which bled the cream of their army, and was halted at the Mures River, the Bulgarians turned their armies north, while the Austro-Hungarians, with some German troops, counter-attacked in Transylvanian. By the time the year ended, the Bulgarians had occupied all of Dobruja, including the south, which had been snatched by the Romanians in 1913; Wallachia was also occupied (essentially half of Romania).

Bulgaria also connected the Germans to the Turks, allowing the Germans to supply them, and keep them in the war. Despite the insistence of Lloyd-George and other easterners, Germany WAS the 'prop' of the Central Powers.

By 1918 however, the effects of having been at war essentially without respite since 1911, began to take their toll. Supplies, especially food, was scarce, losses in combat in Serbia, Romania and at Salonika were adding up, and to make matters worse, Greece had joined the Entente (now the "Allied and Associated Powers"), and the French Army of the Orient (AotO), under General Franchet-Despery, counted 500 000 men at Salonika. The Allies opened an offensive in September, and although Bulgarian troops (stiffened by German support) fought doggedly, their lines began to give way, and the AotO advanced up the valley of the Vardar River towards Skopje. Bulgarian troops began to mutiny, a mass of them gathering at Radomir, near the capital Sofia, demanding radical reform and an end to the war. Considering that the Germans were in the process of being shit-kicked back through Belgium and Eastern France by the Allied armies, on September 30th, Bulgaria agreed to an Armistice, and Ferdinand abdicated on October 3rd, in favour of his successor Boris III. The Allied armies, meanwhile, continued their advance into Serbia.

I don't know of any good books specifically about the Balkan Theatre in WWI off the top of my head, but some good general histories include:

David Stevenson, "1914-1918" and "With our Backs to the Wall" Peter Hart, "The Great War: A Combat History of the First World War" William Philpott, "War of Attrition: Fighting the First World War"

Hope that helps!