Before Ashenden there was Douglas Perry. Perry's book on VHDL (Second Edition) was pretty well written and handled HDL for synthesis quite well. After Perry, I was introduced to Ashenden's "The Designer's Guide to VHDL". This is by far the must have VHDL reference manual. Ask yourself, what is important in a good reference manual...the answer is well organized concepts, good examples and a thorough index! This book has it all. I have other VHDL books that just get dusty on my shelf, not this one! If I'm not using it, someone else usually wants to borrow it. If your looking for a great VHDL book...pick this one up or borrow it.