Shakespeare did not write in a vacuum, and scholars today confirm that he was heavily influenced by Holinshed's Chronicles, and Plutarch's Lives (properly titled Lives of the Noble Greeks and Romans.)
Now, an amateur and deeply capable scholar — Dennis McCarthy — has, in collaboration with Professor June Schlueter, ferreted out what is likely to prove a powerful third influence on Shakespeare, the writing of one of Queen Elizabeth I's ambassadors to Sweden, George North. An obscure diplomat until now (his Wikipedia entry starts onFebruary 8, 2018 — 4 days ago!) he had an elegant turn of phrase that clearly caught Shakespeare's attention and imagination. The book that sparked Shakespeare's — and now our — interest, was A Brief Discourse of Rebellion & Rebels. Read the New York Times article here.
From Wikipedia: A portrait, supposedly of Christopher Marlowe. There is in fact no evidence that the anonymous sitter is Marlowe, but the clues do point in that direction. Marlowe was 21 years old in 1585, when the painting was made. He was also the only 21-year old student at Corpus Christi, where the painting was later found.
New scholarship from the Oxford University Press suggests that Shakespeare had help from Christopher Marlowe when writing Henry VI, parts 1, 2 and 3. Corpus analysis helps solve the puzzle! More.