Kenneth Muir 
Shakespeare’s Sources [PDF ebook] 
Comedies and Tragedies

First published in 1957. This book finds discovers what the sources to Shakespeare’s Comedies and Tragedies really were, considers the dramatic reasons for Shakespeare’s departure from them and provides many examples of the way in which he made use of his general reading for particular scenes and speeches. Kenneth Muir shows that Shakespeare frequently uses more than one source and sometimes as many as eight.