[Trying to remember a book I once read...]

Unfortunately not..it's not by an author I remember....and I am well read in the sci-fi/fantasy genre...I read it back in the 80's possibly early 90's....but thank you for your time

[Trying to remember a book I once read...]

Can't remember too much about it....except the 'lead' of the story is a young lad...with a coat...that he discovers is lined with small jewels and wires linking them....'magic' is performed by drawing in the available heat ...'he drew the heat from his demesne...feeling the air turn chill..'...demesne, a not much used word, is used often..and then releasing the energy in a controlled manner to conjure 'monsters' to fight his opponent.....a little sketchy on any more details..any help or thoughts would be appreciated