The book centres around Maven, a maid and Reeve, a squire who arrive Rennart Castle on the same day but for very difference reasons. On that same day the priceless Fire Star oes missing and the two have to find it.

"On one level it is about two teenage friends solving a puzzle of a missing jewel and navigating this new world around them and on the other it is actually a story about rebellion, secrets and how small decisions can lead to very big events," Ms Tait said.

Allison Tait is the bestselling author of The Mapmaker Chronicles and The Ateban Cipher.

She co-hosts a podcast called So you want to be a writer?. She said the idea for the book came from a 2017 newspaper article.

"The kicking-off point was a newspaper article about a discovery in a farmers field in England," she said.

"There was a rabbit hole and they went down it and discovered there was a underground cave network that was all carved out - they suspect it may have been used by the Knights Templar."

In her new book Allison draws on inspiration from a number of her favourite authors.

"I've always loved mystery stories - as a kid I was obsessed with Trixie Belden, The Secret Seven, The Famous Five," she said.