Gordon R Dickson 
Dragon at War [EPUB ebook] 
The Dragon Cycle Book 4

Baron Jim is in the thick of it again. Somehow he’s found himself taking on England’s oldest enemy — the French — who have entered into an unholy alliance with a most fearsome horror: the mighty serpents of the deep underseas. Nobody has battled them before; nobody even realised they could leave the sea. And with the serpents as their allies it’s only a matter of time before the French invasion succeeds — unless Baron Jim can stop them.