Arianne Cohen has learned a few things from poring over the sex diaries of 1,500 people.

For starters: relationships are a lot like careers. Sure, some of us work 9 to 5—but others stay home in pajamas all day, eating crackers in bed. It can get messy.

Cohen, 30, is quick to point out that The Sex Diaries Project is a collection, not a statistical survey—either way, it’s a fascinating window into what Americans are doing behind closed doors. And while it may be habit to define relationships in black and white—married, divorced, single, coupled—the reality, we learn, is far more complex gradations of gray.

“They’re wildly inaccurate,” Cohen says of most common notions about American relationships. “And they don’t in any way correspond to what people are actually doing.”

Throughout the book’s nearly 300 pages, we meet a 37-year-old mom from California who has a bundle of suitors and a Jewish fetish; an outdoorsy 31-year-old Oregonian who is obsessed with oral sex; and an asexual filmmaker who doesn’t know he’s asexual. We are introduced to a 27-year-old grad student who cheats on his girlfriend with transsexuals he meets in the Casual Encounters section of Craigslist and an 80-year-old San Francisco grandmother who, as Cohen puts it, “is perfectly content taking her morning coffee with a side of solitude.”

“To me, it’s too much work,” the 80-year-old woman says of dating at her age. “I’m a different generation. The women stay home and take care of family and the man. Heck no. If a man asked me for a cup of coffee, I’d say, ‘Well, there’s the kitchen.’”

There are racier diaries, too—complete with a minute-by-minute staccato of one sexual fantasy after the next. (Each diary entry is time-stamped, so we can see exactly how frequently sex is on the brain.) Yet while it’s the sex that sells it—obviously—the text is as much sharp analysis as it is short-form erotica, with Cohen as an effective guide.

In all, we meet 39 diarists, divided up into three sections: soloists, for whom singlehood is a choice (and blessing); partnered couples, who come in varying shapes and sizes; and polys, people (and partners) who may have started out monogamous, but have come to the conclusion that one person simply can’t fulfill every need.

Cohen may have come to this topic by fluke—her previous book was a celebration of tall people—but she has managed to own it, dissecting each diary, categorizing and sub-categorizing them, and pulling out meaning from each.

In The Sex Diaries Project, we see clearly how sex—for at least one partnered 20-something—can distort real feelings, flooding logic with the warmth of a naked post coital glow. “This relationship is so unique!” one man writes of his girlfriend. “I forget how special our bond is.”

We learn that guys watch a lot (a lot) of porn. One man says it’s given him a false sense of average penis size. Another says it’s caused him to view his wife as a “hole for my personal pleasure.” (Yikes.)

We see how husbands and wives can fill different roles for different people: to the sexy stay-at-home mom, a husband is someone who pays the bills. To a lesbian sex educator, a spouse is a soulmate.

And we learn at least a few good tips for how to tell when a breakup might be on the horizon (at least if you’re reading your partner’s diary). “The diaries are something of a looking glass for understanding relationship stability,” Cohen writes. Red flags: entries harping on the word “should” to describe how things might be. A gap in what the diarist is saying (“I love him”) and actually doing. A lack of rhythm to the couple’s day-to-day actions.

“The happiest people in the book are the ones who know what their priorities are, and they feel like they’re on a path to meeting them,” Cohen says. She’s identified 13 consistent priorities among all the diaries: things like sex, parenthood, financial stability, and romance.

“It’s the people who don’t know what their priorities are who get very angsty,” she says. “And they often blame their partners for this.”

Cohen jokes that the book could be called The Options—a kind of geographic guide for the types of relationships that don’t fall neatly into tidy categories. But her own story is perhaps the biggest surprise of all. A former “card-carrying monogamist,” as she puts it, she is now in an open relationship and newly engaged.

“Slowly over the years, I cherry-picked from the parts of the diarists’ lives that resonated, and I put them into practice in my own life,” she says.

The bottom line? We may think we know what’s going on in other people’s private lives—but it turns out we scarcely have a clue.