Long time ago just like in Nickillus' case I assumed that imagination drawings are done completely from imagination. I had some single book on anatomy/perspective and learned from that but what got me started was just accepting the fact that my drawings will always be far from realism. It was sort of like climbing the mountain with too big rock on my back. I didn't realise I had it and even though it prevented me from reaching the top, I grew some imagination drawing "muscles". With a little bit of additional observation I can run twice as fast though .