The Obsidian Dagger is the second book in the Horatio Lyle series by British author, Catherine Webb. As Horatio, Thomas, Tess and Tate work on their pressure-differential-velocity aeronautic device, their research is interrupted by Lord Lincoln, who insists that Her Majesty once again requires Lyles input into an important case. A ships captain and one of Lincolns agents have been murdered on board ship. As always, Lyle is given little to go on, but he certainly garners more information from the scene and the witness than does Inspector Vellum. Lyle finds himself on the trail of a priest and the contents of a stone sarcophagus, but soon, similar murders occur. It seems the very stones of London are coming to life and nowhere is safe. Webb fills her tale with wonderful inventions and fantastic happenings: underwater breathing apparatus, underwater lights, earth tremors, jumps from buildings, flights over the city, a frozen Thames, a great battle, a stone dagger and even a cameo from a youthful Arthur Conan Doyle. Webb expands her main characters a little and brings back one of the characters from the first book for a major role. Nursery rhymes are a connecting theme throughout. This is another enjoyable read and young fans will eagerly await the third book in the series, The Doomsday Machine.