I have just discovered the  Google Books Ngram Viewer. When you enter phrases  it displays a graph showing how those phrases have occurred in a corpus of books (e.g. in “English”, “American English”, “British English”, ”English”,  “English Fiction”, “French”) over the selected years. I tried comparing Rosemary Sutcliff, with Geofrrey Trease, Georgette Heyer, JK Rowling, and Terry Pratchett, in the US and UK. This clearly shows the rise and decline in attention to the books of Rosemary Sutcliff.