The Dover reprint of my 1970 text Elementary Number Theory: an Algebraic Approach is available directly from the publisher. Because it's Dover it's a mere $14. No need to go to Amazon to save two dollars.

Here's the new Preface.

The red points in the cover image identify the primes in the Gaussian integers - the ring of complex numbers with integer coordinates. The green points correspond to 2 and the ordinary integer primes of the form 4n+1. Fermat proved that just those primes are the sums of two squares, so they factor in the Gaussian integers. (Theorem 14.5 on page 31).

If you have a copy of the original edition you will want to know about the errata. If you buy the new one you'll find this list there.