"I think things along these lines are the future for books," Jo Piazza, the ghostwriter of "Marriage Vacation," and author of five books (a sixth is slated to publish this summer), tells CNBC Make It. Though character Brooks is listed as the author on the IRL "Marriage Vacation," it's Piazza's handiwork.
Given that publishers' book sales for trade (consumer) books were flat in 2016, according to the Association of American Publishers, and the Adult Books category saw a 2.3 percent decline, riffing off a popular TV show like "Younger" could be one solution to drumming up excitement around novels.
"The fact that you're competing for eyeballs on all other kinds of content, this is how you can make a book that really stands out, that you can get readers to know that it's out there, and to get them excited about it. So I think finding these kinds of tie-ins is very important to the book industry at this point," says Piazza.
Books that are pegged to popular TV shows also have the potential to create new readership as well.
"These fans might not have read a book otherwise, or they might not be big readers, but they're going to come to this book ["Marriage Vacation"] so it's a win-win for both of us," Simon & Schuster's Christine Pride, editor of "Marriage Vacation," tells CNBC Make It.
"In this competitive media landscape, those are the kind of edges that you're trying to leverage."
Simon & Schuster did not yet have sales numbers, but reader feedback suggests fans are responding: "As a big fan of 'Younger,' I was excited to hear that this book was becoming a reality," writes one Amazon user of the book. "I love the idea of having an opportunity to engage in the plot line of one of my favorite shows in an new, unconventional way. I hope this is the first in a long line of fictional novels from 'Younger' coming to life."