1) I oftentimes notice more flaws during the second sitting.
I did this for Ruined by Amy Tintera. I blew through about 60 percent in one day/sitting and was really enjoying! Then I took a break to start Outlander by Diana Gabaldon and went back into the fantasy two days later. I saw one flaw after another! There was a lot that was lacking in the book that I'm not sure if I would've picked up on if I didn't take a break from it.
2) If I still love a book after the second sitting, then it's probably really good. Right?
That's where the second sitting test comes in! If I can still enjoy a book on two separate days and possibly in different moods, I take that as a good sign. It means I have a longer lasting connection with the characters, story, whatever. Plus, two sittings isn't that long of a time. To mean, that's two separate days or a sitting early in the day followed by one late at night. If I go into a second sitting liking a book much less, it's probably a good thing. Then I can see that's it's not as great as I initially thought and can write a more accurate review.
3) But...should there always be a second sitting?
Not at all! I'm not going to stop reading books in one sitting because of my observations, but I may spend some time imagining if I would like the book as much if I spent a longer amount of time on it. Also, some books are better read all at once. Some thrillers are probably better because you stay in the zone (ha, this is a funny way to put it), which could lead to a more exciting experience. Like many things, it just depends on the book.
Does the time it takes you to read a book affect what you think of it? Have there been books that you really liked in the first sitting, but liked much less after a break?