Robert Musil's 'The Man Without Qualities'. Not quite WWI, but relevant to the last days of the Habsburg Empire. Gives a good idea of what was going on then in the Habsburg Empire and why it would do what it did regarding Serbia.
Jaroslav Hašek's The Good Soldier Švejk. I had a chance to read this in the original Czech so don't know about how good the translations are. But it's a great novel set during WWI. This one also takes place on the Eastern Front.
Thomas Mann's 'The Magic Mountain'. Given that WWI actually interruped the writing of this book and that Mann then rewrote it as the war was going on, this would be a good book also.
Edit: Just saw you edit. Now you know the full title.