Even if the quotes are not always required the do not harm => use them all the time.
BUT you should not use Application.StartupPath
for such a task.
You can use something like the following:
var dir = Path.Combine(Environment
.GetFolderPath(Environment.SpecialFolder.ApplicationData), "MyProgram");
if(!Directory.Exists(dir))
Directory.CreateDirectory(dir);
var path = Path.Combine(dir, "books.zpl");
In the startup path you will NOT have write privileges if you are not an admin and the application is installed to "program files".