The title is Isaiah's name, which seems to check out—seeing as how he's the main central figure in the book and all (aside from God, that is). Since Isaiah's character speaks most of the words in the book, even if they're actually God's words (that is, everything but the narrative parts), this makes a great deal of sense. Even if there were really three or more writers or prophets all going under the name of Isaiah, their collected prophecies together were edited into the form of one book relating the prophetic vision of one character, which we call Isaiah (since that's what all the contributors to this book—regardless of who they actually were—called themselves).
By the way, The Hebrew name "Isaiah" means something like "God saves." Lucky name, huh?, since the whole book is basically about God's saving powers. It's like naming a baseball player Homer, or a ballet dancer En-Pointe.