I'm just a casual birder, but I like the Sibley Guide a bit better than the Petersen. The Kaufman guide is reputed to be the most user-friendly for someone new to the pastime (it uses photo's rather than drawings). I'd use an app as a backup to a good guide, but not rely solely on an app.

“Perfection is Achieved Not When There Is Nothing More to Add, But When There Is Nothing Left to Take Away” Antoine de Saint-Exupery