Unbound: The Conscience of a Republican Delegate makes a powerful case that delegates to the Republican Party convention are not bound to vote for any particular candidate based on primary and caucus results, state party rules, or even state law. Co-authors Sean Parnell and Curly Haugland document nearly 240 instances at past conventions in which delegates invoked their right to vote their conscience.
"All that matters are rules, and the RNC’s rules, according to Haugland — who has pored over them with painstaking attention to detail — offer a surprisingly large amount of leeway when it comes to how the 2,472 Republican delegates must act in Cleveland come July." — Rolling Stone, May 11, 2016
"Incredibly, Republicans at the highest level can’t quite dismiss Haugland’s arguments. Even last week, three days after Reince Priebus declared Trump the presumptive nominee, the party chairman couldn’t quite bring himself to dismiss the possibility that the convention could nominate someone other than Trump." — Politico, May 9, 2016
For the wonkish political nerd, this is a very well researched book, documenting the history of the Republican National Convention. It clearly lays out the case that the right of delegates to vote their conscience on any issue at the National Convention is a basic fundamental principle of the Republican Party, and has been upheld in 39 out of 40 national conventions (the 1976 convention being the only exception).
It also goes into great detail about the Rules of the Republican Party, how they're drafted, who's responsible for drafting them, how they're organized, and when they are in effect. Each convention body adopts its own rules, as well as a set of rules governing the call of the next convention, and rules to govern the Republican Party. Rules adopted by one convention cannot bind the next convention, and serve as temporary rules only for the next convention.