First things first, that's a trickier question than it seems because 'Ikebukuro West Gate' isn't one single novel series with a clear, linear order like some other franchises. The core story is actually the manga 'Ikebukuro West Gate Park,' which I'd argue is the essential starting point.
You're probably coming to this from the anime, right? The anime adapts arcs from the original manga by Ira Ishida and Sena Aritou. The prose novels by Ira Ishida—there's a bunch, like 'Ikebukuro West Gate Park: The Novel' and several others—they exist more as companion pieces, exploring side stories or fleshing out events mentioned in the manga. They're not a sequential prequel/sequel chain you need to follow from Book 1 to Book 10.
My personal take? If you want the main plot about Makoto and the G-Boys, start with the manga volumes. Once you're hooked on that world, the novels offer these cool, gritty, standalone dives into specific characters or incidents. Trying to read the novels first without the manga foundation would feel confusing, like walking into a movie halfway through. I grabbed 'I.W.G.P.: The Novel' after finishing the manga, and it gave some neat backstory on the Color Gangs, but it didn't feel like a 'next step' in a sequence.
So, there isn't a strict reading order, just a recommended entry point.