Based on the information provided, it seems that Carol Jenkins and her mother initially visit Mr. London to look at his collection of books, which suggests an interest in literature or possibly a research project. Upon their second visit, Carol discovers something unusual.
To answer the question effectively, you would typically need to know more details about the nature of the unusual discovery. This could range from finding a rare book, uncovering a secret about Mr. London, or stumbling upon something unexpected within the collection.
Without additional context about what the unusual discovery entails, it's difficult to provide the best answer. If you have more information or options regarding the nature of the discovery, please share them for a more tailored response!