First of all, I must say what a curious question. I cannot think of a time I ever came across a question which refers to the "eighth" most important event in a novel.That being said, allow me to address the question.
When examining the novel Stones in Water, by Donna Jo Napoli, the most important events in the novel can be defined in two different ways. First, the eighth most important event can be defined by using the chronological order of the happenings in the text. Second, one can categorize the events by importance only. Regardless, "important events" in any novel will differ for each reader given the fact that not all readers will identify the same events as being important.
Now, onto the answer. The eighth most important event in the novel can be identified as the time in the novel where the eating of sausage by Roberto is discussed. Roberto decides to eat sausage on Fridays. This is important to the novel because Roberto is, essentially, ignoring his culture's ideals by eating pork.
Keep in mind that this is a completely subjective answer. Other readers may find a completely different event in the novel as being the eighth most important.