The official iOS 9 manual. No, really. And it's free.
Yes, Apple does publish manuals for some of their products. They don't, however, publish a print manual. Instead, they offer free e-books in their iBooks store, which can be read on iOS or Mac.
For a full listing of manuals that Apple publishes (previous versions of iOS, Final Cut, etc.) visit their section of the iBook store.
If you want a more traditional print manual, you may wish to try Phone: The Missing Manual.