This book is to fantasy writers what Marx' manifest is to communists. It will give you a better understanding of fantasy as a mythical journey.

I HEARBY DEEM THAT JOSEPH CAMPBELL'S HERO WITH A THOUSAND FACES REQUIRED READING FOR ANYONE WHO WISHES TO WRITE FANTASY!!!

Yes, it's just that good. I HIGHLY recommend it because of what RoberII has said and more. Campbell introduces something called 'The Keys' which you can look to for a guide if your character is going on a quest. You can also look at this circle he's made with the keys that you can apply to other stories and myths!