A married couple with a mutual love of Costco have written a book about the retailer – and they have also discovered many tips about navigating shopping there over the years.

David and Susan Schwartz wrote "The Joy of Costco: A Treasure Hunt from A to Z" together, with Susan describing it in a joint FOX Business interview on Thursday as a "love letter" to the retailer. The self-published book hit stores – including Costco – in mid-September. 

The book features a broad range of information about Costco and various morsels related to global operations in an "A to Z" format.

The idea of writing it stemmed from their love of Costco and arose in 2016, not long after David’s completion of a biography and as he was "looking for other books to write," according to David.

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While walking in New York City, Susan "turns to me and says, ‘Hey, I have a great idea. Why don’t we write a book together about Costco? We love it, we love everything about it, we can find out all about how they do what they do, and I’m sure Costco members would be interested in reading about it,’" he recounted.

The Schwartzes said creating the book took seven years.

At the beginning, Costco "had no interest in having a book written," Susan said, calling the company "inherently modest." 

"It took us two years to meet the surviving co-founder Jim Sinegal and the CEO Craig Jelinek. It took us then three years to convince them we were writing the book whether they liked it or not," said told FOX Business. 

The company eventually provided them with behind-the-scenes access after realizing they were going to move forward with the project either way, according to Susan.

The couple estimated they made stops at 225-250 warehouses globally along the way. They also clocked 220,000 miles.

In late September, Costco reported its warehouse count stood at 861. Just under 600 of those were located in the U.S. or Puerto Rico, while the company identified the remaining ones as being in 13 other countries.

David told FOX Business they "don’t have a favorite Costco but there are some Costco stores that are really quite remarkable." The couple specifically mentioned the one in Santa Fe, Mexico City, with a rooftop park; the smallest in Juneau, Alaska; and the largest in Salt Lake City, Utah.

"They’re always the same but always different," Susan said. "About 40% of the product is different regionally."

The Costco-loving Schwartzes have picked up various tips for navigating the retailer’s warehouses, which they said typically has a "curated" inventory of 3,800 types of items.

"The first thing is, Costco is committed to early in, early out so if you’re looking for Halloween costumes, do not go there on October 15 because they’re probably all gone," Susan told FOX Business. "Back-to-school is not, you know, the end of August, it’s in July – that kind of thing. Valentine’s Day is way before everybody else because Costco gets in and they get out."

Susan said there are services at Costco that some people aren’t as familiar with, such as their hearing aids center, where she said testing was free and she got the devices for significantly less than other places. The couple takes advantage of their pharmacy services and rents cars through the company, too. 

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Another tip, David said, was that shoppers should go through all the warehouse’s aisles .

"The inventory is always changing, and you never know what you’re going to find. In addition, the pricing may change from time to time, they may have rebates or they may have special markdowns," he explained.

Costco members should also be on the lookout for the "Death Star" asterisk that sometimes appears in the corner of price labels at the retailer. It "means the item is going to be discontinued either permanently or temporarily," Susan said.

She said buying a couple of the items you like that have the asterisk can ensure you have it and "sends a message to the home office that this product is still viable."

At the end of its fourth quarter, Costco counted a total of 127.9 million membership cardholders.

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This week, Costco revealed Jelinek plans to step down at the end of the year as part of a "long-standing succession plan." COO Ron Vachris will take over the job at that time.

Shares of the retailer finished out Friday’s trading day at just under $553.