Let's prolong the list with:

1. Eric Schiller "How To Play The Scotch Gambit" (Chess Entreprises, Coraopolis PA 15108, 1992)
2. Andrew Soltis "Winning With The Giuoco Piano And The Max Lange Attack" (Chess Digest, Inc., 1992)
3. George Botterill "Open Gambits: Italian & Scotch Gambit Play" (Batsford, 1986)

...as well as...

4. Boris Alterman "The Alterman Gambit Quide: White Gambits" (Quality Chess, 2010)
5. Glenn Flear "Starting Out: Open Games" (Everyman Chess, 2010)
6. Vladimir Zagorovsky "Romantic Chess Openings" (Batsford, 1982)
7. John Emms, Glenn Flear & Andrew Greet "Dangerous Weapons, 1 e4 e5" (Everyman Chess, 2008)

...and as a part of a black repertoire...

8. Eric Schiller & John Watson "Survive & Beat Annoying Chess Openings: The Open Games" (Cardoza Publishing, 2003)
9. Igor Lysyj & Roman Ovetchkin "The Open Games For Black: A complete black repertoire with 1.e4 e5 against everything except the Ruy Lopez" (Chess Stars, 2012)
10. John Emms "Play The Open Games As Black" (Gambit, 2000)

...and also...

11. Stefan Djuric, Dimitri Komarov & Claudio Pantaleoni "Chess Opening Essentials Volume 1: The Complete 1.e4" (New In Chess, 2007)
12. Dragan Barlov & Milos Jovicic "Modern Chess I: 1.e2-e4, White Is Better" (Kaisa Marktrade: The World Of Chess, 1994)

Not sure about Chess Openings for White: Explained.  It has a reputation for introducing the Scotch Gambit quite well but also for ignoring key moves for Black (John Watson wrote a particularly critical review of the book).

Tim Harding wrote a couple of online articles on the Scotch Gambit:
http://www.chesscafe.com/text/kibitz74.pdf
http://www.chesscafe.com/text/kibitz75.pdf
...the Two Knights with d4:
http://www.chesscafe.com/text/kibitz115.pdf
I think Harding covers the Scotch Gambit line 4...Nf6 5.e5 pretty well, but is a bit too pessimistic about White's chances after 5.0-0.  For the 5.0-0 line, Stefan Bücker has written a couple of good articles:
http://www.chesscafe.com/text/kaiss14.pdf
http://www.chesscafe.com/text/kaiss53.pdf

I have also provided some analysis and discussion of the Scotch Gambit here:
http://tws27.50webs.com/chess/scotch1.html

Another book to consider is Jude Acers and George Laven's "Italian Gambit and a Guiding Repertoire for White- E4!", for although it focuses mainly on the variation 1.e4 e5 2.Nf3 Nc6 3.Bc4 Bc5 4.d4, it also covers most of the key lines that can arise from the Scotch Gambit.

I was actually wondering the same thing. Anyone have any book recommendations that cover the Scotch Gambit? 

I'm used to playing this as white from a Two Knights Defense move order, but I think the Giuoco Piano and Hungarian are boring, so I'm thinking Scotch Gambit lets me avoid both of those. I just want to be prepared for other variations I'll have to face if black doesn't play Nf6 to transpose into the 2N line that I know.