Verilog Tutorial
The Introduction to Verilog tutorial written by John Sanguinetti is a self-study course for learning the Verilog Hardware Description Language. There are nine chapters in the course, and each chapter has a number of topics and subsections that you visit by moving around in hypertext. The tutorial includes searching features and a test to help you evaluate your learning.
For an introduction to digital design using Verilog, see Digital Design: An Embedded Systems Approach Using Verilog by Peter J. Ashenden (Morgan Kaufmann Publishers, ISBN 978-0-12-369527-7).