The Guild Handbook of Scientific Illustration
http://www.gnsi.org/join/handbook.html

I just got this today, this book is unbelieveable! If you are interesting in doing exacting drawings, paintings, sculpting, museum dioramas, murals, etc. you will learn mountains from this phonebook sized hardcover.

I got a copy for $70 off the cover price off amazon cos it had a barely noticeable warp on the dust jacket.