Dragonshadow by Barbara Hambly is currently 99c in the US
This is the second book in the series and is published by Del Rey and gets some mixed reviews
https://www.kobo.com/us/en/ebook/dragonshadow
https://www.amazon.com/Dragonshadow-.../dp/B002IPZHA8
Synopsis
Lord John Aversin--with the help of his mageborn wife, Jenny Waynest-- has fought and defeated two dragons, earning the title of Dragonsbane. But there are creatures more terrifying than dragons. Demonspawn from a dark dimension have learned to drink the magic--and the souls--of mages and dragons alike, turning their victims into empty vessels. And now they've stolen John and Jenny's mageborn son, twelve-year-old Ian.
In desperation, John seeks the help of the eldest and strongest dragon: Morkeleb the Black. But the demons have allies, too: a vast army poised to plunge the Realm into civil war. In the coming struggle, Morkeleb will sacrifice what he values most. Jenny will question everything she trusts and believes in. And John will embark on a perilous quest for the only things capable of defeating such powerful demons--even more powerful demons . . .