Good Books...

So I was wandering about on the internet looking at apocalyptic literature stuff and I ran across a quote in a Frontline article (follow the "apocalypticism" link) that made me wonder a bit. Prof. Paul Boyer of the University of Wisconsin, in reference to Tim LaHaye and Jerry B. Jenkins' Left Behind books, wrote "They're very readable. They're very well written." Really? Is that so?

I recently had the...well I guess I'll say opportunity to read Dan Brown's best-seller The Da Vinci Code. This book is also apparently considered a good book - it sold a hell of a lot of copies at the very least.

Here's my question. If these are considered good books, what exactly is it that constitutes a bad book?

Update: I thought I was alone in my disdain not only for the content, but also for the presentation of The Da Vinci Code. It would appear that this is not the case...see here.