G Kitson Clark 
The Critical Historian [PDF ebook] 

Originally published in 1967, this book analyses the method by which historical evidence is built up and compares the nature of historical proof with that of other disciplines such as the law and natural sciences. It examines an extraordinary series of forgeries and distortions from the False Decretals to the biographies of Lytton Strachey, as well as discussing how an historical reputation such as that enjoyed by Judge Jefferies was created.