But but but, you splutter. The Rule says No headhopping! The Rule says Stay in one character's viewpoint in each section. These books break the rules.
Yup. The Rules are there, and writers break them all the time. But that doesn't make the rules less valid. You need to understand the reasoning behind the rules, and then you can merrily break them.
The rules are there to make life easier for the reader. That's all. Readers often find headhopping (ie switching from one character's viewpoint to another within a scene) confusing or distancing. Confused or distanced readers stop reading. That's why it's inadvisable.
However, if you set up a multi viewpoint scenario from the beginning (as both Lively and Nicholls do) then the reader is prepared. For example, the opening page of Family Album alternates viewpoint from paragraph to paragraph: ABABA. Once the reader has realised this they can follow the story.
The trouble comes if you are starting out as a writer. The chances are you don't understand why the rule is there. I've come across many new writers who can't see that they're headhopping, They're confused, the writing is confused, the reader is confused.
Exactly the same is true for flashback. Many new writers don't realise they're doing it, many don't understand why the rule is there. They're confused, the writing is confused, the reader is confused. Plus, it often slows down the action, doesn't add new information, goes over old ground.
Learn how and why the rules work, and then, when you know what you're doing and the reader isn't getting confused, you can do what you like. Headhop at will. Flashback away. Readers don't read with a checklist beside them, but they want a smooth journey through your story. If they have to fumble around to check on who is speaking or where exactly the characters are in time then they'll stop reading.
Serve the reader. That's the ultimate rule.
PS Mind you, there is another reason why I advise unpublished writers to follow the rules. Nicholls and Lively are both established writers. They're not sending off their first 50 pages and trying to find an agent, or sending short stories out to competitions or magazines. You probably are. It's a very competitive world (in case you haven't noticed). You don't want to give anyone reasons for rejecting your story and, like it or not, headhopping or misplaced flashback could easily be a reason.