Navigating the 'Solo Leveling' reading order can be a little confusing because of the different formats. The core story started as the Korean web novel 'I Level Up Alone'. The easiest way is to start with that original web novel, either from the beginning or by finding a reliable fan translation. That gives you the foundational story, the internal monologues, and the full scope of the world-building that sometimes gets condensed elsewhere.
After the novel, you can move to the manhwa (the digital comic). It adapts the novel faithfully for the most part, but with stunning artwork that really brings the dungeons and fights to life. Some character moments are streamlined, though. Reading the novel first made me appreciate the manhwa's visual interpretation more, rather than feeling like I was missing details. There's also an anime now, but it's a very condensed version—fine for the spectacle, but the novel and manhwa have much more depth.
Beyond the main story, there are side stories that came after the novel's conclusion. They're essential if you want closure for certain characters, especially Cha Hae-in and Sung Jin-Woo's family. I'd save those for absolute last, after you've finished the main novel or manhwa arc. Jumping into them early would spoil the ending of the primary narrative.