R. Austin Freeman 
Great Portrait Mystery [PDF ebook] 

The National Portrait Gallery is the opening setting for this delightful mystery of theft and fraud. A painter copies diligently from a watercolour one morning when an enigmatic musician suddenly appears and causes mayhem with his musical interludes, hopping from one picture to another and giving a remarkable rendition of different songs. But while the curator follows him around trying to call a halt to the musical spectacle, the copyist replaces a watercolour masterpiece and makes an infamous escape. Who is the mysterious musician? Who is the mysterious copyist? And what has happened to the priceless watercolour?