Let $K$ be an algebraic number field and $\pi_K(x)$ be the number of prime ideals in the ring of integers $\mathcal{O}_K$ with norm $\le x$. In 1903, Landau showed that $$\pi_K(x)=Li(x)+O_K(x\exp(-c_K\sqrt{\log x}))$$ for some positive $c_K$ dependent only on $K$. The original paper (Neuer Beweis des Primzahlsatzes und Beweis des Primidealsatzes) is in German and I have been unable to find an account in English.
Cojocaru and Murty quote the result in their book An introduction to sieve methods and their applications but don't provide a reference. Apparently Montgomery and Vaughan quote the result in their book Multiplicative number theory I. Classical theory. (pp. 266-268), however I have yet to access it.