In the book Vreeland has great fun imagining the decisions that went into the painting, the doubts, the false starts. I purchased the book on the enthusiastic recommendation of a friend with whom I'd seen the famous work.
However, I wasn't able to stay excited about the book. I made it throught the first chapter before running into nonstandard punctuation of a non-restrictive clause. A non-restrictive clause (which could also be thought of as "unnecesary," at least in terms of the grammar of the sentence) takes commas. The commas signal that the information is "extra" information rather than necessary information to identify the subject.
An easy way to understand the difference is this:
If I say
My brother, who lives in Switzerland, has five cats.
the implication would be that I have one brother and that he happens to live in Switzerland. In this case "who lives in Switzerland" is bonus information.
But if my brothers all live in different countries, the phrase "who lives in Switzerland" distinguishes John from all the others. In this case it's a restrictive clause, meaning a necessary part of the sentence that restricts the definition of John. Thus it doesn't take commas. It's not extra.
My brother who lives in Switzerland has five cats.
My brother who lives in Italy has five daughters.
My brother who lives in England is too poor to have anything.
Vreeland's confusing sentence near the beginning of Chapter 2 is "He used to be welcomed anytime, by Jeanne or her mother who stuffed his cheeks with sweets, but that was last winter."
Vreeland's unintended implication is that Jeanne has several mothers, but this one is responsible for stuffing [Auguste]'s cheeks with sweets.
In a similar example at the bottom of the page, Vreeland uses a comma before the first "interrupter," a phrase that interrupts the grammar of the sentence, but fails to use a comma after the phrase. She writes:
Once, just this once he'd....
instead of writing
Once, just this once, .....
It is quite common for writers to make the mistake of using only one comma where they needed two or where they didn't need any at all. To help myself remember this rule, I think: unnecessary (non-restrictive) phrases get commas. We could take those phrases out and still have a perfect luncheon at the punctuation party.