Number Fields - Introduction

Daniel A. Marcus, "Number Fields", Springer-Verlag.
Jürgen Neukirch, "Algebraic Number Theory", Springer.

I recommend Marcus' book. Despite the ugly typesetting, its contents are very easy to read. Concepts are clearly explained, and there is a lot of motivation to go through the material. (I made these notes partly because I wanted to read these notes in a less ugly form.)