"I'll keep an eye on you, my chicken."Biggles is James "Biggles" Bigglesworth, ace pilot and adventurer.
Biggles is a boys' adventure series of widely varying quality, but the good ones are legitimately good. Johns flew in WWI, and the series features realistic depictions of aviation and aerial combat from that time period, though the series later extends into WWII and post-war adventure. If you click on the author tag, you will find a post with more context on him and Biggles.
The other thing about Biggles is that he uses some truly unique epithets and exclamations. Maybe they were real slang? But no one else in the books says stuff like "Suffering rattlesnakes!
Biggles Flies East features the fantastic opening hook of Biggles getting mistaken for a different British pilot, a shady malcontent, and offered a job spying and flying for the Germans. Next thing he knows, he's been inserted as a double agent - a job which he isn't qualified for and which he dislikes very much, but which he turns out to be surprisingly good at.
Biggles has an extremely nervewracking time on every possible level. Not only does he need to maintain his cover, gather information, and regularly report back to his own side without getting caught, but the Germans quickly discover his piloting skills and want him to fight. For them. Against his own side. And that's not even taking into account his nemesis Erich von Stalhein, a spookily perceptive intelligence agent who seems to sense something off about Biggles...
This is basically an adventure story with elements like spies being given signet rings as identifiers, but if you take that stuff as a given, the book is surprisingly well-plotted, with some excellent twists. It plays the spy/undercover aspect to the hilt and is legitimately tense. It's extremely full of incident, and leans into the premise to an astonishing degree. Whatever your favorite undercover/imposter trope is, this book probably contains it.
Biggles is a very human character in this installment. He's uncomfortable with his job all the way, is terrified a lot of the time, and has a lot of compassion and fellow feeling for other soldiers, even if they're the enemy. He and von Stalhein have a fascinating relationship, very heavy on mutual respect and slashy vibes.
This makes the unintentional innuendo-by-language-drift moments even better. I'm talking about stuff like,
"Not so fast, my cock," Biggles ejaculated.I don't recall von Stalhein appearing in the books I read as a kid, which is too bad as one of my absolute favorite things is honorable enemies who would be friends if they weren't on opposite sides of the war, and sometimes are friends anyway but they must still meet in battle. I am pretty sure I got this from the Mahabharata, which I also read as a kid.
Von Stalhein is the archetype of the noble and honorable enemy. You see this type of character a lot in WWI fiction, especially when focused on aerial combat, and he's a very appealing version of it. His introduction in this book is GREAT.
He appears in that context in the WWI books, then makes some appearances in WWII books where he's completely different and basically an evil thug, then vanishes from the series only to reappear when it's safely post-war, where he picks up from where he left off in the WWI books and continues to be the dashing and honorable enemy/crush.
Here's what I think happened. W. E. Johns created a classic WWI archetype, then the books became smash hits, then he continued them into WWII and realized that his honorable enemy character did not work in that setting TO SAY THE LEAST, tried writing him as actually terrible, found it deeply depressing, then jumped him ahead in time to a setting that suits him better. At least I assume that's what happened. If so, I feel for Johns.
For my continued fannish enjoyment, I shall consider that von Stalhein was a double agent in WWII and that's why he was so out of character, which would actually make sense with his roles in both earlier and later books.
These books are very hard to find in the US and I obtained my copy
at this odd site.You can also find a few Biggles books and several Worrals books at
Faded Page.
Worrals was Biggles' female counterpart. I'm excited to read some of the Worrals books, as those have previously been impossible to find (as opposed to difficult).
General content note for Biggles books: Levels of racism/colonialism range from zero to a line or two of mild stereotyping along the lines of "The Arabs are a noble warrior race" to holy shit I didn't know you could pack that much racism into a two lines of dialogue. In general, I recommend avoiding any books with titles like
Biggles in Australia. 
