"Five Days in Philadelphia: The Amazing 'We Want Willkie!' Convention of 1940 and How It Freed FDR to Save the Western World", Perseus Books, 2005.
The finest thing that can be said about the book is that although I knew how both the republican convention and presidential elections of 1940 turned out, the convention (especially) made for compelling reading. I say this especially since we no longer have conventions like that.
I was forced to ask myself if this was a good thing or not. I'd have to say that in the short term, the answer is no. The freedom of the state delegates meant, at least in 1940, that there was no chance of a victor on the first ballot, and a wide number of candidates to consider. It was the job of the delegates to consider, and the job of the voters to choose wise delegates.
Another amazing thing, hard to remember, was that Hitler was simultaneously invading many countries _during_ the convention of 1940. France signed its official surrender papers.
Many prominent republicans had no interest in the war, the standard excuse being the chronological proximity with the "War to End All Wars" (a chemical weapon war).
President Hoover, who seems to have been making a comeback, was saying that Hitler had basically won, and what America needed was a man who could do business with Hitler, and who hadn't alienated him.
Another fine feature of the book is that the author, as a young teenager, was present in 1940.
I really have no knowledge of any other books about conventions, but you could do a lot worse than read this book to get the feel for them.