You can read everything of Zapffe's that's ever been translated into English tonight[1]! If you want more pure pessimism you shouldn't leave out Ligotti's "Conspiracy Against the Human Race", which also cover's Zapffe a bit. Ligotti is not a traditional philosopher but he makes some great observations about the relative rarity of true pessimism in Western philosophy.
It's worth noting that Nietzsche's real work was to find a solution to the inevitable pessimism and nihilism that he feared would inevitably threaten Western culture. Schopenhauer and Zapffe go straight off into pessimistic view of human existence (Schopenhauer deriving some humanistic "we're all in this together" ethics at moments and Zapffe saying we should stop breeding and let the flame burn out). Nietzsche still ultimately rejects nihilism.
It's worth noting that Nietzsche's real work was to find a solution to the inevitable pessimism and nihilism that he feared would inevitably threaten Western culture. Schopenhauer and Zapffe go straight off into pessimistic view of human existence (Schopenhauer deriving some humanistic "we're all in this together" ethics at moments and Zapffe saying we should stop breeding and let the flame burn out). Nietzsche still ultimately rejects nihilism.
[1] https://philosophynow.org/issues/45/The_Last_Messiah