When you installed Fritz 15 it would have put the book in the standard folder which is at:
In Fritz do FILE/OPEN/OPENINGS BOOK then navigate to the folder above and click on the book and it will open. Now in the Fritz game window, to the right of the board under "notations + opening book" if you click on the openings book you can see the book there.
That book is OK if you just want to look generally at openings but if you want a stronger book eg. for engine play you need one that has been tuned. Best way to find out the strongest book is to look at Seat Canbez SSCT Book Tournament. Not sure if you are familiar but he runs huge book tournaments where the best books compete against each other over around 1000 games. So he uses identical engine and conditions and the only difference is the book used so you can see which is strongest. The last complete tournament is at this link with the results:https://sites.google.com/site/computerschess/scct-book-cs-13
Most top books are private and the authors don't always share but some are shared freely. The one that came in second place in the last completed tournament so a strong book has been openly shared you can get from here (est.ctg). And no the winning book has never been shared before you ask !sendspace.com yj4t4w
New tournamwnt just started is here:https://sites.google.com/site/computerschess/scct-book-cs-16
Best to keep an eye out in this forum and other chess forums to see if anyone is sharing the books from these tournaments.
If you don't know how to install a book - after download you will find 3 files - the book name with .ctg .cto and .ctb endings (spmeimes also an ini file). Copy and paste ALL those files into the folder C:\users\your name\documents\chessbase\books then in Fritz when you open a book like I described above the new book will be there.
By the way am guessing you may not be expoerienced in computer chess so if you go to this link there is very good help on using the Fritz 15 functions:http://help.chessbase.com/Fritz/15/Eng/
Hope this helps you.