Popular books? Go to Amazon/Ebay's top 10 sellers or something.

As for your criteria:

Written very well and clearly, preferably even entertaining.

That's pretty much what every good book would have.

Does not require the reader to write anything (e.g., practice problems) or do anything beyond just reading and thinking, except perhaps on very rare occasions.

I find this criteria to be of dubious merit. But perhaps it's because I'm not a native English speaker and I'm used to checking up words or phrases that aren't intuitively clear to me even after I tried to analyse a few sentence back and forwards.

Cannot be "heavy" reading that requires the reader to proceed slowly and carefully and/or do lots of heavy thinking.

Like GEB?

Can be understood by anyone with a decent high school education (not including calculus). However, sometimes this requirement can be circumvented, if the following additional criteria are met

You'd probably need a specialized book in order for that to apply.

We'll need to know what a "decent high school education" is, though. Humourously, of course.