Sapper 
Final Count [PDF ebook] 

When Robin Gaunt, inventor of a terrifyingly powerful weapon of chemical warfare, goes missing, the police suspect that he has ‘sold out’ to the other side. But Bulldog Drummond is convinced of his innocence, and can think of only one man brutal enough to use the weapon to hold the world to ransom. Drummond receives an invitation to a sumptuous dinner-dance aboard an airship that is to mark the beginning of his final battle for triumph.