The blurb--he blurts out--always the blurb.
Most bookstores I go into have the books displayed with the spine facing out(except for new releases on that rack),
I usually look first for something or someone new and then slip the book out and look at the blurb.
If the blurb is good I read the first couple of pages and if that's good...I eventually notice them turning the lights out and realize I' m not in the library.
I follow similar practice on line and I've only once or twice had amazon disappoint me with no sample.
That much said, a good cover is helpful in determining how I'll display it at home to tempt the other readers in the family.