Most bought and least read?

What makes a 'best selling' book? Who reads it? Does it have literary value? Does it matter if it does not have literary value?
I own a copy of Stephen Hawking's, "A short history of the universe" - and I have read it. I do not claim I understood all of it. Physics is not something that greatly interests me. However I endured three years of badly taught high school physics and remembered enough to make some sense of the book. I wanted to find out what the fuss was about. I still do not know what the fuss was about. Professor Hawking once nearly ran over my toes demonstrating his (then) swish new state of the art wheelchair to me. That was rather more memorable than the book. A physicist later suggested that the book had to be one of the 'most bought and least read'. I suspect he was right. You needed to know something about physics and mathematics. The ideas are big. I think Professor Hawking has since modified his ideas but they are, in all likelihood, bigger than before. He's a big thinker. All the same it was not, I am certain, the ideas in the book that were selling but the fact that the author was viewed as a rather romantic figure -although his life is anything but romantic.
Most 'best-selling' books however appear to be quite the opposite. I see them being read on the train on occasion. I sometimes feel I should be embarrassed at being caught reading a book about language death or the revival of Gaelic or a new book from the children's shelf instead of the latest bodice ripper. Other readers probably think I am posing but I enjoy these things. Bodice rippers, war books, westerns and certain types of science-fiction/fantasy hold no interest for me. I have yet to read a book by Colleen McCullough. I tried to read Thornbirds - and it bored me. I have not yet read Tim Winton's Dirt Music. I started and got sidetracked by books I found more interesting. There are other 'best sellers' and 'best selling' authors that must do something for other people. They buy them. They appear to read them. I do not. There must be something wrong with me.
I prowl along the library shelves. I look at the little pink stickers which say 2009 and haul the book out. The blurb on the back or on the inside cover starts to tell me about a convoluted set of relationships. In fairness I open the book. Nothing happens on the first page. I delve a little deeper. There is a long and apparently meaningless conversation over four or five pages. Nothing happening there. The book is thick, probably 120,000 words and perhaps even more.
I can't be bothered. It might be a 'best seller' on the New York Times list or some other apparently respectable 'list'. It is not on my list.