Harry Potter and the Prisoner of Azkaban
We only hear about the series' Super Villain second-hand in this novel. Voldemort appears in flashbacks where he's murdering Harry's parents, or as the topic of discussion among various groups in the wizarding community. When he is being discussed, words like "most evil wizard ever" are used and people tend to adopt a tone of fear and dread. So, while we don't get any Voldemort in person in this book, his presence is still very much felt. And that presence is one of lingering fear (note how most characters refuse to say his name, even a dozen years after his downfall). It's interesting to pay attention to the characters that actually speak Voldemort's name: Harry is one, Dumbledore is another, and Sirius Black also says it. You have to be a pretty brave and bold individual to say his name out loud.
So, given how important Voldemort is to the series, what's the deal with his absence here? Well, this novel is really more about the people connected to Voldemort during the first war – those who fought against him, those who died fighting him, and those who opted to join his side. We don't see the present-day Voldemort in this novel because the focus here is largely on the past Voldemort, as he was at the height of his power during the first war. You can read more about Voldemort's absence in the theme sections "Memory and the Past" and "Fear."