There are many web sites and books on VHDL.  Unfortunately, most of them are not very useful for students learning digital design. 

Among the best resources I have found are two books:

• Chapter 4 of "Digital Design and Computer Architecture" (2e) by Harris and Harris, and
• "VHDL for Logic Synthesis" (3e) by Andrew Rushton.

Both are available as on-line e-books through the BCIT library and the library has a hard copy of the first edition of the Harris & Harris book.