What Makes a Book Popular?

I've been posting a lot about my recent activities. Life has been hectic with visits to stimulating writers groups and a host of searching interview questions both online and on various radio stations. I still have quite a few events to blog about. I haven't told you how much I enjoyed talking to Thames Valley Writers, or how I felt being interviewed live on Radio 91.8 FM (another live radio interview coming up next Saturday), or being approached by Woman and Home Magazine (where we talked about blogging!), or spending time signing airside at Heathrow Terminal 5 (where I saw ROAD CLOSED displayed at No. 7 on the WH Smith's Travel Bestseller Charts), hearing that my books are displayed at No. 4 and No. 6 on the Bestseller Chart at Waterstones in Bedford, or the thrill of receiving an email from award winning author Sam Millar who's just read ROAD CLOSED "and really loved it" (Sam won the prestigious Aisling Award for Art and Culture, the Martin Healy Short Story Award, the Brian Moore Award for Short Stories and the Cork Literary Review Writer’s Competition. His best-selling memoir, ON THE BRINKS, has recently been acquired by Warner Brothers... and that's just a taster)

But I thought it was time to return to some serious discussion about writing. Even I can see that there's only so much personal news I can blog about without becoming... how shall I put this?... repetitive... (OK, boring...)

One of the questions I was recently asked (Hayes FM) was:
"CUT SHORT launched in the summer and sold so fast your publisher had to reprint after two months. What elements in the book do you think made it so popular?"

That's a tricky one. What is it that makes some books grab our attention while others leave us unengaged?
For those of you who missed the radio broadcast my answer at the time went something like this.
"My books are plot driven so readers read on to find out what happens. But it is character that interests me. People fascinate me. Perhaps the popularity of my books is due to a combination of exciting plots and convincing characters." I went on to say that of course many authors achieve that, so I can't account for my success with CUT SHORT reprinted 3 times in its first year and ROAD CLOSED already off the starting posts, with its first few reviews pretty positive. So far a Top 50 Reviewer on amazon has said: "The characters are believable and I really like Geraldine... I also liked the way everything dovetailed together so that while you're reading you get those light bulb moments when a piece of the jigsaw slots into place..." Another reviewer says "tense and gripping... with an exhilarating climax that you don’t see coming until it is too late... Geraldine is a gifted, strong and likeable character." Sam Millar wrote of ROAD CLOSED that it is "a gripping, fast-paced read, pulling you in from the very first tense page and keeping you captivated right to the end with its refreshingly compelling and original narrative...Geraldine Steel is a complex and highly driven character, with multifaceted feelings of contradiction and nuance." Jeffery Deaver also mentioned plot and character, now I come to think of it, when he described CUT SHORT as "a seamless blending of psychological sophistication and gritty police procedure. And you're just plain going to love DI Geraldine Steel." In fact most of my reviewers cover both plot and character, so perhaps it is a combination of strong plot and convincing characters that is winning fans for my Geraldine Steel series.

What elements in any book do you think make it popular? It's an important question for an author - but is it an impossible one to answer?