Harry Potter and the Prisoner of Azkaban
We get fairly limited Slytherin interaction in this book. The ones we hear about, Crabbe, Goyle, Pansy Parkinson, and various Quidditch team players like Marcus Flint, tend to just shadow after Malfoy. Aside from the usual anti-Gryffindor behavior, the Slytherins' real moment to shine comes in the epically dirty Quidditch match they played, though there they are depicted more as a block of jerks and not as real individuals. Which is probably how Harry sees them.