Frankly there are some many amateurs on the Internet playing the King's Gambit that it's getting tiresome. However I believe there is a book by Thomas Johansson, King's Gambit for the Creative Aggressor.
Kings Gambit
A good book for a brief description of most of the variations is "King's Gambit" by Korchnoi and Zak. The book is mostly dry analysis, best kept for reference.
A rather more readable effort was written by McDonald "The King's Gambit". A nice mixture of concepts, thoughts, analysis, games and stories.
There is a new book out King's Gambit by Nikolai Kalinchenko (Chess House publisher) that looks pretty good--I also like Etrin's two volumes Play the King's gambit which while somewhat dated gives some good overview (it usually can be found on e-bay fairly cheap)--for the King's Bishop Gambit I use The Fascinating King's Gambit by Johannsson ( self published available through the Internet)--and of course as mentioned Gallagher's book.
I have the Gallagher and the Korchnoi/Zak books on the KG as well as one by Christiansen : The KG for White ... a couple of co-authors in that , dont know how much of the work is actually Christiansen's .
The long awaited and much anticipated book The King’s Gambit by John Shaw (Quality Chess) is scheduled for publication in Dec/Jan (we've heard that before but I'm an optimist!) It's been several years since we had a major new work on the King's Gambit and the theory has advanced by leaps and bounds. I'd hold off buying any of the older books unless you find a really good deal - better to save your money for the Shaw book.
the Gallagher book use to be one of the best but it's 13 years old now so i don't know if anything better has come along since. 'Winning with the king's gambit.' The nice thing about this book (one of the nice things anyway) is that Gallagher plays the king's gambit at GM level events. To my knowledge there are authors out there who write on specific openings but don't even use them. Gallagher is not that guy. Incidently, David Pruess can probably recommend a good book too. I think he has a good record with the lines. Gallagher mainly covers the king's knight gambit. I'm not sure what a good bishop's gambit book is.