Where does history end, and storytelling begin? And when the mysteries of the past step forward to the present day, how do we separate fact from fiction?

I have received rather a lot of correspondence from readers of The Lost Library over the past ten months, asking the inevitable questions over that dancing, moving, confusing line that separates fact and fiction. ‘Which of the historical facts you related are actually true?’ ‘How much did you make up?’ ‘Did things really happen like that?’

My standard answer has always been the same: ‘If you can’t tell, then I’m not going to tell you.’ It is in creating a situation where fact and fiction are so intermingled that we cannot fully separate them or tell them apart, that historically-minded fiction becomes interesting. At the very least, to me.

But sometimes, it is not only the readers who ask these questions.

The process of writing such stories is often as gripping, and contorted, as reading them – a fact I’ve become ever more aware of in the finalisation of The Keystone, which will launch this August. This book has allowed me to dive back into a world I know well, and love: ancient Egypt – though not quite as ancient as most imaginations are prone automatically to travel. Millennia after the pyramids had already started to decay, yet still almost two millennia before our own day, the sands of the Egyptian deserts were home to activities that still confound the world. Sects with strange rituals hid libraries away in caves; mystical rituals challenged the norms and power of empires; and secrets began to be whispered from one ear to another – secrets that remain hidden even in our day, though we have never fully escaped the influence of the secret-keepers.

Where is the edge of ‘fact’ when we draw ourselves towards a world of dark-corners, traditions never committed to print, and written works so cryptic that generations of modern scholarship cannot agree on what they mean, or even just who wrote them?

And if the earliest decades of a new millennium were marked out by secretive groups that might have existed, that might (or might not) have been eradicated, but whose influence could still be traced out centuries later in parts of the world that had no connection to this haunting land of origin, is it beyond belief that their influence might extend even into the present day?

The world, as I explore it in The Keystone, is one where these ambiguities are transformed from mild and idle curiosities into the stuff of definitive, immediate relevance and panic. It is a world where a manuscript seen by a thousand scholars might say something none of them have ever seen; where alchemy and history are not as tidy in their distinctions as modern scholarship likes to keep them; where chants of peace and cries of war often speak in the same language. And where nothing is ever truly committed to the past.

Is this fiction? Is it fact?

Or is even that, like so much else that cries out for our attention in the face of a compelling mystery, a little too firm a distinction for a world that so rarely deals in black-and-white?