Every now and again, you pick up a book with no great expectations, settle down to read a few pages and discover that it has quietly sneaked under your defences. Before you know it, you are emotionally invested in the characters, wondering what is going to happen next and becoming increasingly reluctant to put the book down.

The Valley of Lost Secrets by Lesley Parr is one of those books.

It is a little gem.

Set in the steep, green valleys of South Wales during the Second World War, it tells a story that manages to be comforting and exciting at the same time. There is mystery, friendship, betrayal and adventure, but there is also a gentleness to the storytelling that makes it feel like one of those children's books from a bygone age.

It is a slow burner, and I mean that as a compliment.

There are no fireworks going off on every page, no characters running around at a hundred miles an hour while the author desperately tries to keep up. Instead, the story takes its time. It allows the characters to breathe and, more importantly, allows us to get to know them.

The writing has an old-fashioned quality to it, in the very best sense. It reminded me of the books I grew up reading, where children could have adventures without needing a mobile phone, a computer or a billionaire's secret underground bunker. Somehow, life seemed more interesting when all you needed was a pair of muddy shoes and a rather questionable sense of judgement.

It is difficult not to compare The Valley of Lost Secrets with books such as Carrie's War and Goodnight Mister Tom. That is pretty lofty company, particularly for a debut author, and comparisons can sometimes do a book more harm than good.

But here is the thing.

The Valley of Lost Secrets holds its own.

It deserves to sit alongside those books.

The story begins with Jimmy and his younger brother Ronnie being evacuated from war-torn London and sent to a South Wales mining village. It is a huge upheaval for Jimmy. He has been taken away from everything he knows, deposited in a strange new world of steep hills, unfamiliar people and a mining community that feels a million miles from London.

Even the name of the village is enough to cause problems. It contains more L's than Jimmy has probably encountered in his entire life. I suspect there are Welsh place names specifically designed to test the limits of the English tongue.

Ronnie, however, takes to their new surroundings rather better. He seems to adapt to village life with the sort of irritating ease that only a younger brother can manage when his older brother is struggling.

Jimmy finds it much harder.

Thankfully, the brothers are billeted with Mr and Mrs Thomas. They are not exactly the conventional couple and, rather sadly, are treated as outsiders by some of the villagers. Yet there is warmth in their home, and slowly the boys begin to settle into their strange new life.

Then they discover a skull in a tree.

This is not something that happens very often in the average evacuation story.

Most children might discover a bird's nest, a lost football or perhaps a dead mouse. Jimmy gets a human skull. It certainly gives a new meaning to the phrase "interesting things to find in the countryside".

Naturally, questions follow.

Who does the skull belong to? What happened to them? And why was it hidden in a tree?

The mystery becomes the thread that draws the story forward, but what makes the book work so well is that the mystery never overwhelms the characters. It develops alongside Jimmy's own story.

And Jimmy is really at the heart of the book.

He is not one of those irritatingly perfect fictional children who always knows what to do, says the right thing and charges heroically into danger while the rest of us sit back and admire them. Jimmy is frightened. He is uncertain. He misses home. He struggles with the changes that have been forced upon him.

In other words, he feels like a real boy.

As the story progresses, Jimmy gradually begins to understand the village and the people who live there. He makes friends, learns whom he can trust and starts to find his place. His emotional journey is just as important as the mystery surrounding the skull.

There is a lovely balance here between the excitement of discovering the secrets of the valley and the quieter story of a boy learning to cope with a world that has suddenly been turned upside down.

And then there is the setting.

The Welsh mining valley is almost a character in its own right. You can picture the steep hills rising around the village, the rows of houses clinging to the valley sides and the ever-present influence of the mine. There is a strong sense of community, but also the feeling that everyone knows everyone else's business.

Which, of course, is both comforting and potentially disastrous.

Because that means that keeping a secret becomes rather difficult.

And this is a book about secrets.

It is also about friendship and betrayal, about prejudice and kindness, and about discovering that people are rarely quite what they first appear to be. Some of the characters are immediately likeable, while others take a little longer to understand. Just like real life, really. Thankfully, nobody arrives wearing a badge saying I am the villain. Although it would make things considerably easier.

What I particularly loved was the patience of the storytelling. The book slowly builds, just as Jimmy slowly settles into his new surroundings. By the time the mystery begins to unravel, you care about what happens because you have become invested in the people involved.

And then, suddenly, you reach the final few pages.

I wasn't expecting quite how emotional they would be.

There are books that entertain you while you are reading them and then disappear from your mind as soon as you close them. This isn't one of those. The Valley of Lost Secrets stays with you. Not because it is loud or spectacular, but because it has something much more valuable: heart.

To say I loved this book is probably an understatement. It grew on me gradually, almost without me noticing. By the end, I cared deeply about Jimmy and his journey, and I found myself wishing there were another hundred pages waiting for me.

I would happily recommend this to readers aged seven to eleven, but I wouldn't stop there. This is one of those rare children's books that can be enjoyed across generations. I can imagine it being read by a child today and then being picked up by their parent or grandparent afterwards, who suddenly finds themselves remembering the books they loved as a child.

It would also make a wonderful class read for children studying the Second World War. It brings that period to life through the eyes of a child without turning the history lesson into a history lesson.

The Valley of Lost Secrets is warm, atmospheric, mysterious and deeply touching.

It feels like a modern book that somehow wandered in from another generation.

And perhaps that is why I enjoyed it so much.

There is something wonderfully reassuring about finding a new book. It reminds you why you fell in love with reading in the first place.

A gem of a story. A wonderful debut. And, I suspect, a modern classic in the making.