From the Fighting Fantasy series.
Update: thanks to Andrew [see his post] he's been able to identify where the following three illustrations came from. Much appreciated! All I can add is, well, one was from the FF series.
pumps his wings full of blood, and a dash of magic, and they swell and spread, like a butterfly. After all, it would be really dangerous to have fully developed wings in a cave. Don't believe me? Check out Renaissance paintings, they knew.