References: Grothendieck Groups of Spaces and Varieties
Solution 1:
Question: "Good people! I'm asking for some good references here! I need to find the Grothendieck groups for a couple of varieties, and I'm struggling with figuring out how to make this work..."
Answer: If $X$ is any scheme and $E$ is a rank $e+1$ locally trivial sheaf on $X$, it follows there is an isomorphism (the "projective bundle formula")
$$K_0(\mathbb{P}(E^*)) \cong K_0(X)[t]/(t^{e+1}).$$
In particular for projective $n$-space $\mathbb{P}^n_k$ you get
$$K_0(\mathbb{P}^n_k) \cong \mathbb{Z}[t]/(t^{e+1}).$$