Michael Gilbert 
Into Battle [PDF ebook] 

Luke Pagan is stationed in Portsmouth to keep an eye on Zeppelin flights. He discovers a British Naval officer doing exactly the same. Seemingly, there is a German spy ring operating with a view to directing the Zeppelins so they can keep an eye on the British fleet, although at this time its identity is unknown. Moreover, at the start of World War I, two ships are blown up and the head of counterintelligence orders Luke to carry out surveillance on the suspected British Officer. This in turn leads to the uncovering of an Irish terrorist plot bent on throwing off British rule by any means possible. The Irish have infiltrated the dockyard. It is clear there is an international conspiracy at work which turns out to be run by a German: Krieger. Luke finds himself in a race against time to identify all of the espionage and terrorist activities — or risk Britain losing the war.