title: how to add an object to an observable of array
body: <p>I have an angular project with a service called BookService.</p>
  
  <pre><code>private books: Subject&lt;Book[]&gt;;
  
  getBookList(skip:number = 0,limit:number = 0): Subject&lt;Book[]&gt; {
    return this.books;
  }
  
  addBookToList(book:Book) {
  }
  </code></pre>
  
  <p>this service help me to monitor the changes in a list of books, I also want to be able to add a single book to the list using the addBookToList function but I dont know how to add a single book the this observable (the subject-observer)
  is it possible to add a single book</p>