If I were to recommend one book, out of the many I've read, to someone who wanted to know what a film director does, this would be it. Minghella is unparalleled in the lucidity and ease with which he dissects the director's role in making a film.
The book is compiled from a series of interviews, revolving around different aspects of the filmmaking process, from the general to the specific. However, there is a unity to the tone that makes it a totally coherent work, instead of the cut-and-paste job you might expect.
It's true, Minghella has only made 4 films that he feels warrant discussion. (The fifth, Mr. Wonderful, is saved for a brief chapter at the end, detailing what went wrong with it.) Still, he mines more advice and explanations about his creative role from those four films than most other directors are capable of finding in a whole career's worth.
Obviously the fact that he's an accomplished playwright means that he's more at ease with language than most filmmakers, and every page is full of fascinating surprises, as he manages to put complex ideas and aspects of his creative process into words.
I should only add that this isn't a book I would necessarily recommend to everyone. Minghella demonstrates such a ferocious intelligence that this would likely be over the head of the average casual film buff. This is not "Directing For Dummies." Instead, this is a dense, challenging, but endlessly rewarding read for those who want to be informed and have their perspective broadened.