It's by Peter Dickinson and in it he details the chemistry and physics of dragon flight. With the aid of a "flying brick" he explains that dragons cannot use normal flight systems because of their size. The maths is a bit complicated, but basically if you double the size of a dragon you have to quadruple the size of its wings in order to gain lift. If you take that to its logical conclusion the wings become impossibly large.
So he concludes that historical dragons (Eldritch is a modern dragon, of course, and doesn't follow these rules) must have been almost weightless and gained lift by producing hydrogen inside their bodies. He gives the complex chemistry involved in the interaction between bones and digestive juices that could make that happen. This also explains the firey breath thing because they need to burn off excess gas in order to land.
The book gave rise to a wonderful film by the same name (loosely based on it to say the least) that featured the magnificent voice of James Earl Jones as the baddie.
I've just found out that second-hand, paperback copies of this book are changing hands for £75 each. I wouldn't part with my hardback copy for the world!