NBC

Fans of “The Office” know that childhood wasn’t so great for Erin Hannon, the emotionally vulnerable Dunder-Mifflin receptionist played by Ellie Kemper. But things worked out better for Ms. Kemper, who not only isn’t an orphan and wasn’t raised in a foster home, but has a creative younger sister, Carrie, with whom she is about to write a comic novel planned for 2012.

On Wednesday, the Three Rivers Press imprint of Crown Publishing said it had acquired the book, tentatively titled “Monday Sessions,” in which Ellie and Carrie Kemper chronicle a fictitious Manhattan psychiatrist named Dr. Amy Rice through her diary entries and patient files.

Asked in a telephone interview how long she and Carrie had this project up their sleeves, Ellie Kemper said: “Oh my gosh. Well, we’re not sure. We hope that it ends up coming out of our sleeves.”

The Kemper siblings are already used to working together at “The Office,” where Carrie recently joined as a staff writer, and occasionally writes lines that emerge from her sister’s mouth.

(“It’s a great sense of power over her,” Carrie Kemper said. “If she makes one false move, she’s off the show.”)

Dating back to their childhoods in St. Louis, the Kempers worked together on plays and home movies (one of which, “The Man Under the Stairs,” has made its way onto YouTube).

Carrie Kemper was also a precociously sensitive diarist at the age of 5, keeping a journal that is still the talk of the family. “I say a lot of really, really controversial things,” she said. “I know that I do spell Dairy Queen D-A-R-E C-W-E-N. So that was pretty revolutionary.”

“I sort of want to publish that,” said Ellie Kemper, who has also written for The Onion and McSweeney’s. “We won’t publish that, but this is the backup.”

Around Christmastime, the Kempers said, they started brainstorming more seriously for a book project they could work on together.

“We were really thinking, what could appeal to most of America?” Ellie said. “And we thought: a series of psychotherapy sessions on the Upper West Side of Manhattan.”

The Kempers said they wrote a 20-page proposal for “Monday Sessions,” and have a year to write the manuscript, which they say they can do in their downtime during “The Office.”

“Ellie keeps promising me that she has a lot of extra time between scenes,” Carrie said.

“Carrie has to write all day,” Ellie added. “I have so much down time on set. I keep assuring Carrie I’m just going to write it up while I’m taking breaks. I might be talking big, I don’t know.”

“If worse comes to worse,” Carrie said, “I figure I can write Ellie off the show for a season, just say that Erin is off writing a book about an Upper West Side therapist. Then it becomes a weird art-life confusion.”

“Monday Sessions” was acquired by Suzanne O’Neill, senior editor at Three Rivers Press, from Erin Malone of William Morris Endeavor Entertainment, an agent for Carrie and Ellie Kemper.