Get sum of property values from an observable array

This is why you're getting unexpected results for Observable.count
To get the array lenght of the results, you need to do, something like this:

BookStoreService.ts

...
getBookStore() : Observable<Bookstore> {
   this.bookstore$ = this.http.get(...)
      .map( (response:Response) =>  response.json() )
      .map( response => response.bookstore);  // optional depends if JSON payload you want is wrapped inside some other container
   return this.bookstore$;
}

Component.ts

  ...
  bookstore$ : Observable<Bookstore>;
  bookstores: Bookstore[];
  numberOfBookStores:number;
  constructor(private bookService:BookService) {}

   ..
  this.bookService.getJobs()
   .subscribe(
     bookstores => {this.bookstores = bookstores;
                    this.numberOfBookstores = this.bookstores.length;
                   },
     err       => { this.bookstores = [] as BookStore[];
                    // Caters for 404 etc error - be sure to be more robust in final code as you may want to indicate types of error back to user. 
                  },
     ()        => {
                  }
   );

Update:
If you only need to loop through the list in yourHTML template, then then defining the bookstores array as a property would not be necessary. I did this to illustrate how to get the size of the returned collection of bookstores.

You can use this type of syntax:

<tr *ngFor="let bookstore of (bookstore$ |async) as bookstores; 
            trackBy bookstore?.id; let i = index">
  <td>{{bookstore.numberofBooks}}
<tr/>

You can find out more about:

• *ngFor trackBy, even, odd, first, last here.
• Using Async pipe for entire block of html template with AS here

Furthermore have a look at libraries like Lodash and Underscore for summing count of number of books. I've not used Underscore myself. Here's a simple example to get you started.

If you want to get more adventurous have a look at this Functional Programming in Javascript Tutorial