Bhasker J. A Verilog HDL Primer 2ed 1999

Textbook in PDF format

Explains the language through simple examples.
Explains the syntax of language using commonly-used design terminology.
Explains the behavioral style, the dataflow style, and structural style in detail.
Concepts of delay and timing are clearly explained.
Testbench writing is made easier by providing several examples.
Many hardware modeling examples have also been provided to make this an excellent reference