Lightest setup to sell only digital content with no physical products? I'm looking for recommendations on how to setup Drupal 7 with the fewest modules so I can charge users to Buy Digital Content (Book Pages) on my site.

I will eventually have several books, and users will need to pay for each one to read (view) them. There are no files to be downloaded here, I will actually try to prevent users from copying the content; they should only have access to the Book.

I looked at the recommended Drupal Commerce, but it seems very much like overkill (many unused features like shipping, packing, inventory) for what I want to do. Not sure it can (easily) handle Digital Content.

If there is no simple solution, can this be easily done in Drupal Commerce?
You could get by using Drupal Commerce with just the core modules, some custom Rules, and your preferred payment gateway module. The modules and features you pointed to (shipping, inventory, etc.) and the ones you didn't point out have nothing to do with Drupal Commerce itself. They're all optional contributed modules and don't have to be added if you don't need them.

It's possible you saw a demo site running Commerce Kickstart 2.x. That is a distribution of Drupal that includes Drupal Commerce and a wide variety of contributed modules to make it function more like a robust eCommerce application out of the box. At its core, Drupal Commerce is much lighter weight, and I think you'll find its compatibility with Rules combined with the Content Access / Access Control List modules to be very helpful in selling access to digital content on your site.