A lot of people will tell you to stick to the order, but I kind of think it depends on what you're looking for. If you're a completist who needs every character motivation and every subtle hint about Anna's childhood unpacked, then yeah, reading them in the intended sequence is pretty much non-negotiable. Tessa puts those little breadcrumbs in the early books that pay off way later, and you'll miss the gradual build of Hardin's... well, I won't call it redemption, but his change. Skipping around might leave you wondering why certain reactions are such a big deal.
That said, I accidentally started with 'After We Fell' because my library hold came in first. I was confused, sure, but I was also immediately sucked into the melodrama. It was like jumping into a soap opera mid-season – you pick up the dynamics fast because, let's be honest, the core conflict is pretty consistent. I went back and read the earlier ones after, and it was a different experience, more about understanding the origin of the wounds rather than being surprised by the fights. So, essential for a deep, layered understanding? Absolutely. Essential for enjoying the messy, addictive drama? Not really.
The new companion books and epilogues are a different beast. Those you should definitely read in order relative to the main story, or they'll spoil major plot points. Reading 'Before' after the main series was a weirdly melancholic experience, knowing how it all turns out.