This is a survey of the literature on the Morphy Gambit, an excellent weapon for White that begins 1.e4 e5 2.Bc4 Nf6 3.Nf3 Nxe4 4.Nc3. It is also a nice sharp reply to the boring Petroff. I present the views from the following sources with many key improvements.
Bird. Henry Bird. The Chess Openings Considered Critically and Practically. Dean & Son 1877.
Fletcher. L. Elliott Fletcher. Gambits Accepted. Routledge & Kegan Paul 1954.
Janjgava. Lasha Janjgava: The Petroff. Gambit 2001
Yusupov. Artur Yusupov: The Petroff Defence, Olms 1999.
BB. John Watson & Eric Schiller. Big Book of Busts. Hypermodern 1995.
BCO. Garry Kasparov, Ray Keene et al. Batsford Chess Openings. Batsford 1982
BCO2. Garry Kasparov, Ray Keene et al. Batsford Chess Openings. Batsford 1989.
CBPB. ChessBasePowerBook 2007
CDe4. Bernard Cafferty & David Hooper. Complete Defence to 1 e4. Pergamon 1986 (3rd ed.)
ECOx. Alexandr Matanovic et al. Encyclopedia of Chess Openings. Chess Informant. 1977-present. x = edition number. The first two editions didn't have any significant differences.
GCO. Eric Schiller. Gambit Chess Openings. Cardoza 2003.
MCOx. Modern Chess Openings. Various editors. (x = edition number)
MCOE. Anatoly Karpov et al. Modern Chess Openings Encyclopedia.
NCO. John Nunn: Nunn's Chess Openings. Everyman 1999.