Some good suggestions on this thread, however, many of them are not novels but rather factual accounts - novel: a fictitious prose narrative of book length, typically representing character and action with some degree of realism. So anyway, pedant mode off, here's three unusual recommendations, in that they are all, on the face of it at least, children's books, or at least books aimed at young adults. Of course you shouldn't let that put you off, after all, being one of Heinlein's 'juvie' novels, Starship Troopers was aimed at early teen readers, and we all know what a cool book that is, if a little controversial. So, one from each realm of war, air, land and sea...
If you're interested in WW1 and aviation, then as far as novels are concerned, I would seriously recommend reading W.E. John's very early Biggles novel - Biggles Learns to Fly - for whilst it is true that in later novels, Biggles books were very much intended to be 'thrilling takes' for adolescent boys, the early Biggles novels, and particularly Biggles Learns to Fly, was written before Johns had really established his young audience, so it is actually quite adult in its approach (fairly grim in places) and is certainly not afraid to cover subjects such as death and what we'd now call post traumatic stress disorder. It's also notable for being more than a little autobiographical; W.E. John's was in the RFC, and later the RAF, although unlike his fighter pilot fictional hero, Johns was a bomber pilot, which of course required no less skill or bravery. Thus what we have in the early Biggles novels, is the rare combination of a skilled writer who is writing about a subject with which he was intimately familiar, particularly since Johns was in fact shot down, survived the incident, was a POW, though sadly, his gunner did not survive the encounter. Many of these experiences make it into the book in fictionalised form. I like this book so much that I re-read it every once in a while, and I must say that every time I do so, it reminds me of the times when I was a nervous student pilot, learning to fly myself. Seriously, don't be put off by the notion that this is a kid's book, because it really isn't, it's a cracking read.
And speaking of cracking reads, here's another supposedly 'kid's book' which is well worth getting hold of - Cracker!: The Best Dog in Vietnam, by Cynthia Kadohata. This is interesting in that it is written from several viewpoints of the various characters in the book, including that of a young boy's pet German Shepherd Dog, Cracker, who is taken up by the US Army and trained as a booby-trap sniffer dog for service in South East Asia during the Vietnam War, after the boy's father loses his job and cannot afford to keep Cracker. And yes, I know that the notion of writing from an animal's point of view sounds cheesy, but trust me, it is not, it is in fact unbelievably convincing and insightful (I have a GSD myself, and I found it utterly convincing). Again, this is a book which is not afraid to tackle difficult subjects such as death, loss and the like, and not least the sense of loss the boy feels when he has to give up Cracker. It also focuses on how poorly many veterans were treated upon returning to the States after serving in Vietnam, which is why I would hardly call it a kid's book. But perhaps of more interest to people in this forum, is the level of research and authenticity in the military aspects of the book, particularly when Cracker and his handler are seconded to work with a Special Forces unit in the jungle (it came as no surprise to learn that again, much of the book is based on a true story). I actually learned quite a few things I did not know about SF operations before from this book, and trust me, I've got rows and rows of books on the pretty much every war, including Vietnam, so I considered myself fairly expert on it, yet still learned some new stuff. Seriously recommended, although warning, it will make you cry, but not for the reasons you may suspect, however, I won't spoil it for you. Certainly one of the most exciting and innovative war novels ever, particularly for its writing style and intuition into how dogs might perceive us. Even your girlfriend will like this book, but you'll probably like it more. I would go so far as to say that this is the best war novel I've ever read, and I'd put money on you agreeing with me if you do read it. Can be found cheap on ebay too!
The Lame Duck by Richard Armstrong, is again aimed at young adults, and I in fact did take this out from my local library and read it when I was a kid, since it was published in 1961, but I had such fond memories of it, that I sought out a used copy of it (since it is long out of print) and read it again a couple of years ago, and was pleased to note that it was not mere nostalgic memories that impressed me, it is a very good tale and is one of a series of novels which follow the same characters throughout their war service. Armstrong was a radio operator in the Merchant Navy between 1920 and 1937, but worked as a typist in WW2. He started writing novels in 1942, most of which centered around the Merchant Navy convoys in WW2, so his novels are written with much authority and knowledge, and it shows in the finely crafted stories and details. The Lame Duck focuses on a young lad serving aboard a Merchant Navy tanker, when his ship is hit and left behind as the convoy it was part of maintains speed to avoid also being torpedoed, and the remainder of the story focuses on the efforts of the crew to keep their lone ship afloat and make it through with vital supplies, limping along whilst severely damaged and playing cat and mouse with the U-Boats of the Kriegsmarine. A compelling story of heroism from those who had to step up to the plate and find courage to fight the enemy even when they were not in the military, often a part of the war which receives little attention. It's hard to find a copy of it, but like any book which is hard to find, www.abebooks.co.uk nearly always comes up trumps and is well worth bookmarking on your browser if you are seeking a rare out of print publication.
