The riddle of a silver snake (#3). Jones L.

Since Agatha had been accepted into the gatekeepers guild, a secret organization that unravels the most difficult crimes of London, the girl dreamed of the present case. And finally, she had a chance to prove to everyone that she is an excellent detective. A very strange event occurred in the National Gallery: one of the curators disappeared. There is every reason to believe that he was abducted. But why should anyone attack the museum employee? And what does the most famous pictures of the world have to do with it? Agatha does not even know what overwhelming discoveries she is waiting for her!

The third book in the series.