Kraft paper is fine for cheap paperbacks, but I wouldn't ever send a hardback in it. In fact, I don't send PBs that way except to family and friends I'm swapping with - customers get full card packing whatever it is. In my opinion only card gives proper protection to the corners. Very vulnerable are yer corners.
Every book I send gets a tissue layer first, to give the cover a smooth, gentle surface to rub against (ooh er) so it doesn't get scuffed. then corrugated card is cut to size and wrapped tightly around the book, and snuggly folded over and taped at the ends and seam so there is no movement. Then the lot's wrapped in kraft paper for a professional finish and clear labelling. I've only ever had one book arrived damaged in years of sending them, and that was when I first started and was still using jiffy bags. I've only had one book lost in the post, and that was in Italy, where the post is virtually random.
I've lost a few *here* of course, but that's just old age.