Evelyn PM: What I've Been Reading & What Makes Books Hard to Get Through?

I've mainly been reading two books (A Column of Fire by Ken Follett and The Order of Time by Carlo Rovelli) these past two weeks (I forgot to post last week).  

As I mentioned in class a while ago, I've been finding A Column of Fire ("Column") harder to get through than the two prior books in the Kingsbridge trilogy, Pillars of the Earth ("Pillars") and World Without End ("World"), both of which I remember not being able to put down even though they are each about 1,000 pages.

This is pretty surprising for a number of reasons: (1) the Kingsbridge trilogy is relatively new, as in originally World Without End was the sequel to Pillars, and there was no need or even sense of a third book; (2) the two books as a set were a phenomenon both because World was as good if not better than Pillars, which is pretty rare, and because they were way different than Follett's usual spy/conspiracy thrillers, which always sold well (so there was no inherent call for a genre jump); and (3) in between publishing World and Column, Follett completed an entire other trilogy (Fall of Giants, Winter of the World, and Edge of Eternity), all of which were terribly long and incredibly difficult to get through.

It's almost as if someone said to Follett, "Why don't you make those two stragglers into a trilogy?". I'm currently ready to throw my fist through that guy's face … 

So far (I'm about three-quarter through), Column has been a real disappointment. While the plot lines are just as interwoven as they were in Pillars and World, I'm less enamored with the characters; they seem less complex than the Prior Philip, Lady Aliena, Caris, and Jack that I remember from the prior books. The relationships between the characters also aren't as developed, and there isn't the same mysticism or imagery that Pillars and World had in abundance. Maybe I was expecting too much: a third book that met the excellence of the first two?

On the other hand, The Order of Time, which is a lot like Astrophysics for People in a Hurry by Neil deGrasse Tyson, has completely blown me away. For a book that is really an onslaught of total mindfucks (ie. time is unique at every point in space and faster further from Earth's center of mass -- faster in the mountains, slower at sea level), I was expecting to need time to process, but Rovelli talks you through it (a lot like how you'd reason with your drunk friend who is rediscovering how revolving doors work). It's somehow an absolute breeze to read. 

So what makes a book hard to get through, not difficult -- emotionally or intellectually -- but dense? Why is Pillars, which is almost double the length of Column, so much easier to read? How is it that The Order of Time can introduce and explain such complex theories yet be such a quick read? Is it harder to write the easier book?