Everyone knows, when you’re at the airport, you’re allowed to buy any trashy book you want.

Because it’s “for the plane.”

I always have a book in my bag that I brought from home. The one I’ve been meaning to read.  But once I get to the airport I admit to myself that that book isn’t going to be any more inviting once I’m crammed into seat 22F than it would be in my living room.

So, even though there’s no chance I’ll read them all — or maybe even any — once I finish with Elle, Allure, Marie Claire, US, Life & Style, whatever else I haven’t read at the nail salon, and (ha ha, as if) the Wall Street Journal, I load up on options. Usually:

1. one non-fiction book about forming new habits, changing your brain, or becoming a business titan;
2. some novel that’s been on top of the bestseller list for 4 years and now has the “soon to be a major motion picture” cover — which depresses my husband so I have to rip it off; and,
3. if I spot one, please God, a celebrity-on-drugs biography.

Enter Mackenzie Phillips.

As part of my new goof-off-offline agenda, I just started reading her recently published autobiography, “High on Arrival,” which I bought about 4 airplane trips ago. It’s been sitting in the “meaning to read because I paid for it” pile ever since.

The book is a shocker.

The big scandal that made news when it hit the shelves was that Mackenzie had sex with her father, Papa John Phillips.  But that’s not so shocking. Come on, he was from the 60s. What do you expect from a guy who wears caftans and shoots up entire poppy fields in a day?

To me, the shut-yo-mouth get-outta-town surprise is that Mackenzie Phillips was hot.

Now THAT is news to me.  I watched every episode of One Day at a Time and always thought of her character, Julie, as the homely sister. Barbara, played by Valerie Bertinelli, was the pretty one.

Believe me, I’m a lifelong fan of skinny.  Skinny I get. But I did not get her look. She was all gums and bad bowl haircut and terrible dance moves, which you saw every episode in the show open.

It confused me to no end that Julie was constantly juggling dates and phone calls from boys, and had fights with her mother over whether she could move in with them. How was she in a position to move in with a boy, much less go for a soda with one?  There was an episode called “Julie Goes All The Way.” I thought, come on:

Who would want to go all the way with her? I mean, other than Schneider?

I figured the show had an all-blind writing staff or something. But it turns out Mackenzie Phillips was hot and I just couldn’t tell. She had a zillion boyfriends in real life, and slept with Mick Jagger.  He didn’t need to have sex with any fuglies. Even when he was stoned out of his gourd.

I’m still trying to parse this information. I keep flipping open to the smooth photo pages in the middle of the book and looking at the pics from her teenage years, squinting to see the pretty.  No, all I see are the gums.  I still don’t get it.  Maybe it’s like a Shelly Duvall kind of thing.

I’m not being catty. It’s just screwing with my understanding of traditional beauty standards.

Now, see, this book has given me a lot to think about. So is it really that trashy?

Seriously: no, it ain’t Tolstoy. But although I’m prone to book shame — especially when I’m toting around a read like this, or something with Tori Spelling on the cover, or, worse, the stock chick-lit illustration of high heels and cocktail glass — I don’t consider a book true trash as long as it gives good story.

If the writing is truly awful and cliche and contains sentences like, “she was mad as a hornet when she found him cheating on her,” then it’s trash. In which case, I can’t get through it. I keep it with the receipt to return it until the 31-day window has expired, and then I put it down in the basement for a neighbor to poke through while they’re doing laundry.

But if it’s a page-turner, then for me, as a writer, there’s a real takeaway.

Writing doesn’t have to be provocative and “literary” to be worthwhile. You’d be surprised how many books that scream “beach read for dummies on Dummy Beach” can teach you something about smart storytelling.

Got any other good, would-only-buy-it-at-the-airport reads to recommend? Please. Comment away.

Below, the One Day at a Time show open. Tell me Julie doesn’t dance like Axl Rose.