As a Stoic, there are many texts, but I would consider "On Passion" by Chrysippus to be the primary and most important one, maybe next to Zeno's "Republic."
Both texts are lost. However, portions of "On Passion" can be reconstructed through commentaries on it, enough that we can gain a decent enough approximation of what the original book covered.
Chrysippus was the student of Zeno of Citium, the latter of which is the founder of Stoicism. While this makes Zeno to humans what Surak is to Vulcans, most of Zeno's works have not survived and it was Chrysippus who defended them and helped Stoicism grow and continue after Zeno's passing. In my opinion, he might be the most important figure in Stoicism.
I have read quite a bit of commentaries on this text, including debates regarding the ideas it presents. I have also read later Stoics such as Seneca, Epictetus, Musonius Rufus, and Marcus Aurelius and how they interpreted Chrysippus, but if I'm being honest I think a lot of these later Stoics missed the mark, although I think they're still worth reading.
Chrysippus is important because he represents the point in early Stoicism when it was still closely entangled with Cynicism, because this is what Stoicism originally was. At the risk of oversimplifying, Stoicism originated as an internalized and mature form of Cynicism. What set it apart was its focus on dispassion, which is what "On Passion" covers in detail. That's the other major reason that I see it as so central to the philosophy, and the ideas within it remained in ancient Stoicism until its extinction.