I often have writers send me photographs to use for their book cover which they took with their smartphone. Pictures taken with smartphones are fine and dandy to use for Facebook, Instagram, Email, the web.., but for printing for your book cover, not so much! (Ebooks use is fine).
Today smartphones have really progressed in the camera area and have more megapixels than ever before. However, a camera is still better to use when you want to capture that perfect image to put on your book cover or on a page inside your book. High resolution is very important when you want to print images. If an image is poor quality resolution, you end up with a blurry image which looks cheap and unprofessional.
Light sensors in phones can’t compare to how a camera can adjust for lighting, such as a flash.
Stability is also a factor. Smartphones do have stabilizers but they are digital stabilizers and not optical stabilizers like cameras have.
Zooming is much better with a camera too, it has the optical zoom where a smartphone uses a digital zoom and you can get pixilated images as a result.
So, if you are just wanted that quick snap shot to post an image on your Facebook page or send as an email, your smartphone is great for that. But if you are wanting to have a perfect photo to place on the cover of a book or for pages inside a book, or even for posters, a good camera is always the way to go!