The Greatest Books discussion

I was surprised to find this book at the very top of the “non-fiction” meta-list: I confess I didn't even know about it, I don't know anyone who has read it, and my family never had a copy (in fact, I have never seen it).

It is very old (even by best-books-ever standards), and it “sounds” very religious (thus, boring).

Have you read it?

Why does it merit the honour of being in the top spot?
What is so enlightening or innovative about it?
And — ahem — is it as boring as it seems…?