The austrians got into the war because a substantial faction of the leadership, a faction led by their chief general Franz Conrad, thought that a war would inspire national unity in a country that was coming apart and because they, not entirely incorrectly, thought that slavic nationalism in general and serbia in particular were a leading cause of that disunity. Crushing serbia would, to their mind, kill two birds with one stone.

After the war started, two things happened. One, the war almost instantly spiraled into a far larger conflict. Two, Austrian ineptitude in the prosecution of that war led to them rapidly becoming little more than a vassal of germany, which left them little room for wider ambitions. At brest litovsk, for example, the austrians were represented, but they had almost no say in the deliberations and in practice the germans took over almost all of the territory ceded. Had the war ended with a german victory, that settlement likely wouldn't have remained as one sided, but it speaks volumes about austria's total lack of influence at that point in the war.

For the start of the war the best one volume summary is probably Margaret Macmillan's "The War That Ended Peace"