Neptune King and the Mysterious Monk is the 27th book in the series. They're hired by Friar Luigi, a Capuchin monk from a monastery in Devon. Ever since a collection of rare books, diaries and manuscripts arrived at the monastery there has been strange goings on in the night. Each morning they find the library in disarray; the books scattered about or misplaced and sometimes strange noises in the night. When they go to investigate they find nothing and no one. This is puzzling enough, causing some of the monks to believe there is a poltergeist in their midst, but when elderly Friar Matteo is...

Neptune King and the Artifact is the 28th book in this detective series. When Neptune's sister, Mermaid Griffith buys him a gift while on holiday in Italy, she has no idea at the time what trouble it will cause. First, her hotel room is broken into twice and her belongings ransacked, then to her dismay, she is no sooner home than her house is also broken into. The Griffiths stay at his house overnight to stop Mermaid spending all night cleaning up the burglar's mess, but they're woken during the night to an intruder in Neptune's house who manages to get away. Neptune's interest is captured...