I've found that unlike languages like C, HDLs are lacking a lot of in depth tutorial. I know a bit of VHDL and Verilog, but there are so many useful features that are not apparent until you know what you are looking for. How I learned VHDL is through working with someone in the industry doing RTL design, and Google of course.
This is not really a tutorial per se, but it is an extremely useful reference: VHDL Golden Reference Guide , Doulos