What Sells a Book?

I am sitting here and pondering why it is that one book by an author may sell better than another.

Is the writing style different? Not usually. Each writer tends to have a particular style that makes their work recognisable as theirs.

Have they tried a different genre? Could it be their readers simply prefer the writer to stick to one particular theme?

Is the blurb not tantalising enough? Every writer will tell you that the blurb is harder to write than the book!

Or is it the cover? I've heard it said time and time again that a good cover sells the book.

Or is it simply the author's name?

But what is it for you? What do you look for?

If you see one of your favourite authors' names do you immediately snap it up, or do you look at the cover and think, hmmm? Or read the blurb and the story doesn't appeal, regardless of who has written it?

Do you see a cover, like the look and buy it on the strength of that?

Or could it be that the author has bombarded their different social networks with so many "coming soon" adverts that after the twentieth time they've guaranteed you wouldn't touch their book with a barge pole?

Of course, it could simply be the length. If you're used to a writer giving you a tome the length of War and Peace (exaggeration - not that I'm given to that you understand *coughs*), do you see a novella of thirty thousand words and pass it by because it wouldn't satisfy you?

So... what is it that grabs you and makes you go "Oooh... I must have that?" One of the above, or a combination?

What are your thoughts?