Read the book “The Sleepwalkers”. A very informative book about the political conditions that existed on both sides and led to the outbreak of war.
Yes, it really is an excellent work.
That one has my vote as well, although it isn't exactly a primer. Most of the good books on the subject aren't. Christopher Clark, its author, has pointed out that there is more primary source material on the topic than any single scholar could read in a lifetime. I believe it was also he who remarked that the same evidence that leads you to believe the war was impossible will also lead you to believe it was inevitable.
I had the privilege of attending a centenary colloquial on the topic in 2014. The introductory speaker, who was the head of the university's history department, told us that what started the war wasn't really complicated, that he could sum it up in two words: "Nobody knows." I happened to be in one of his classes on military history ("it's intellectual porn," he said, smiling) when he told us there were curses in history, like opening an Egyptian tomb, questions that must never be asked lest the questioner be sucked into a rabbit hole from which there is no return. One of them was "Why did World War One start?" "But since you're already here, you're doomed..."
An awful lot of things had to go wrong simultaneously to produce the multiplier effect that led to an assassination in Sarajevo resulting in an invasion in Belgium. The proximate cause of the war was the Ottoman Empire's withdrawal from southern Europe but events more specific than that clumped together to make the cascade unstoppable; contrarily, there are a number of points at which one key decision could have minimized its scale. Had that car in Sarajevo not taken the wrong turn, for example...