Where do stories come from? Very often from other stories. You read something, which sparks off ideas in your own head. They may be as basic as “I could do better than that”, or you may think of a new approach to take, or you may just want something out there that you yourself would want to read. Robert Muchamore wanted something his teenage nephew would want to read and ended up with the CHERUB series. Bear Grylls and others have done similar.
If you’re really lucky, then you get to play with someone else’s toys. Author Kate Saunders, like me, loved E. Nesbit’s magical stories as a child: Five Children and It, The Phoenix and the Carpet, and (less famous, but in my opinion the best) The Story of the Amulet. And, like me, she did the maths and worked out that the brothers in those books were exactly the right age to go and die horribly on the Western Front a few years later. My imagination at this point went more R.C. Sherriff than E. Nesbit, so it’s probably as well that I never tried to write down what was in my head. She, however, wrote Five Children on the Western Front, which is exactly the story Nesbit would have written, in the same tone and style as the originals, including the witty things that Nesbit had to say about gender and class – which probably went straight over the heads of the target audience, but not of any adults who might be reading the stories to them. She sticks to Nesbit’s style and ethos and still manages to tell a moving WW1 story.
What really makes it work is that it develops the original story. You feel you have moved along rather than just retrodden the same ground. Any novel should really be about the most important thing that has happened to the protagonist(s) at that point in their lives.
This is what I wanted to do with H.M.S. Barabbas, the further adventures of Jim Hawkins after Treasure Island. There are many sequels to Treasure Island and every one I have ever seen is essentially a return to Treasure Island, even though Jim’s last words in the first book are that “Oxen and wain-ropes would not bring me back again to that accursed island”. I thought it would be far more interesting to see what Jim – a young man with a taste for adventure, a fortune in the bank, and a tendency to moral compromise – would do next that did not involve going back there.
Of course, there is the small detail that if your favourite author died less than seventy years ago then their work is subject to copyright. It may still be, if other parties have since got hold of it. You can write what you like in the privacy of your own home, but you may not distribute it; or if you do (for example, J.K. Rowling is known to be very relaxed about fans doing exactly that; other copyright holders are less forgiving) then you may not make money out of it.
This is not a problem. Change all the names, file off the serial numbers and make it your own. Fantasy writer Terry Brooks made his name with The Sword of Shannara, which, with the best will in the world, is essentially Lord of the Rings-lite. But (crucially) he mixed in enough original ideas of his own that he was able to develop a successful fantasy series out of it, with further novels that went in a very different direction.
Alex Rider (and CHERUB) would not exist without James Bond, James Bond would not exist without Richard Hannay … The sequence goes back, probably to tales of heroic chivalric knights, and beyond. They all build on what their predecessors did, but do it differently. There’s no reason why the stories piling up in your head should not be the latest stage in your own favourite genre’s evolution. And if you want a hand getting them out, just ask.