iOS delegation design pattern

func evt_bookModel(bookModel: BookModel, didUpdateCurrentBook book: Book) {
    var index: Int?
    for (key, value) in DataStore.defaultInstance.books.enumerate() {
        if value.id == book.id {
            index = key
            break
        }
    }

    if let _ = index {
        bookView.updateBook(book, atIndex: index!)
    }
}

I see a few problems in this block of code alone.

Ultimately, this could be written far more simply as:

func evt_bookModel(bookModel: BookModel, didUpdateCurrentBook book: Book) {
    for (key, value) in DataStore.defaultInstance.books.enumerate() where value.id == book.id {
        bookView.updateBook(book, atIndex: index)
        break
    }
}

This eliminates a variable, and better yet, eliminates a force unwrap (that isn't even necessary if you just used the if let correctly).

class BookModel {

    private(set) var currentBook: Book!
    weak var observer: BookModelObserver?

    func updateCurrentBook(book: Book) {
        currentBook = book
        observer?.evt_bookModel(self, didUpdateCurrentBook: currentBook)
    }
}

We've got more unnecessary optional problems in this class. There's not a particularly good reason for currentBook to be an implicitly unwrapped optional in this case. It can cause nothing but problems.

There's currently nothing else to this class, so it's unclear why we need to keep a reference to the book at all, but our updateCurrentBook can be changed slightly to do the same thing, not need any unwrapping code, and not need the implicitly unwrapped optional (which again is only going to lend itself to a future Stack Overflow "fatal error found nil while unwrapping optional" question).

class BookModel {
    private(set) var currentBook: Book?
    weak var observer: BookModelObserver?

    func updateCurrentBook(book: Book) {
        currentBook = book
        observer?.evt_bookModel(self, didUpdateCurrentBook: book)
    }
}

Of course, this might be even better if we just used the didSet property observer method on the property itself:

class BookModel {
    private(set) var currentBook: Book? {
        didSet {
            if let newBook = currentBook {
                observer?.evt_bookModel(self, didUpdateCurrentBook: newBook)
            }
        }
    }

    weak var observer: BookModelObserver?
}

But now it seems weird that currentBook could be set to nil and the property observers not notified of that. So here's what makes most sense to me:

class BookModel {
    var currentBook: Book {
        // a willSet probably also makes sense
        didSet {
            observer?.evt_bookModel(self, didUpdateCurrentBook: newBook)
        }
    }

    weak var observer: BookModelObserver?

    init(book: Book, observer: BookModelObserver? = nil) {
        self.book = book
        self.observer = observer
    }
}

In this case, the book property always has a good value.

Or, alternatively, we make it fully optional. It doesn't matter if it's public or private.

Here's the biggest problem with your original implementation:

let fooBookModel = BookModel()
let fooBook: Book = fooBookModel.currentBook // crash, book is nil, it was never set

This is perhaps not how the class is intended to be used, but nothing at all about your code prevents this usage.

My sample prevents the empty initializer and forces this:

let barBook = Book()
let fooBookModel = BookModel(barBook)
let fooBook = fooBookModel.currentBook

Here, the currentBook property can never be nil. And the property observer code is handled in a far more Swift-standard way versus the sort of Swift-weird setter approach in the original implementation.