+ (NSString*)book
{
     __block NSString *bookName;

     dispatch_async(q, ^
     {
        bookName = @"Testing";
     });
     return bookName;
}

q = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, "q");

[SClass startUpBookstore];
label.text = [SClass book];

dispatch_async(q, ^ //BAD_ACCESS here
{
    bookName = @"Testing";
});

+ (NSString*)book
{
     __block NSString *bookName;

     dispatch_async(q, ^
     {
        bookName = @"Testing";
     });
     return bookName;
}

+ (void)setBook:(NSString*)name
{
     dispatch_sync(q, ^
     {
        book = name;
     });
}