That's a very good response, thank you. I get what you mean about if a book has resonated with a number of people it must be doing something right. I should have stressed- and might edit- in my original question is the importance of the fundamental writing itself- the structure, composition, language used, research of the subject matter etc rather than the plot itself. I suppose popularity itself if a win, if it gets people fired up about something.

That said, my comment regarding the greater importance of strong writing for books on 'worthy' subjects comes from a couple I've read that tackle tough, emotive, controversial subject matter, but are written so poorly and researched so badly all I can think is "these authors are charlatans who got cut a book deal purely because of the subject matter". To me this seems like the most shameless of scams and does not give the subject the justice it deserves.