Henry Robbins has been in a dream for 30 years.

“There I was, with my brother Yale, and we were sitting at his dining table – and we decided to go into the real estate industry,” he said. “I wake up suddenly the other day, and 30 years have gone by, and we’ve found ourselves running the largest privately held real estate information and publishing business in New York, if not all America.”

Mr. Robbins paused, and then said, “Do I believe this happened? No.”

His disbelief might be taken with some skepticism, but not the success of Yale Robbins, Incorporated, the company that the brothers founded. It produces an array of directories, newspapers, and guidebooks that generate annual revenues of $8 million. It also sponsors an annual co-op and condo trade show that draws thousands of participants from the tri-state area.

The brothers’ success flows out of a question that every builder, broker, banker, or real estate professional asks: How much floor space does this property have?

“We were the pioneers of analyzing new and renovated residential construction on a dollar for square foot basis,” Mr. Robbins said. “Earlier, analyses were on the basis of dollar for rooms. When I started consulting and providing feasibility studies for builders and bankers, I said, ‘Aha, wouldn’t it be more convenient for the customer to have up-to-date information provided?’ So we started assembling each property with a profile and floor plans.”

Applying that simple technique yields enough information to sustain the Robbins publishing enterprise as well as a popular Web site, Condo-Sales.com. Want to know the price and square footage of any condominium sold in Manhattan? Go to the Web site. Another Web site, MrOfficeSpace.com, offers information about available office space, suite by suite, in 10 states on the East Coast.

Mr. Robbins and his brother – who’s older than him by seven years – also offer a Co-op-Condo Directory. A review of condo sales? There’s the Condo Sales Report. Perhaps an estimate of space available in Manhattan office buildings? The brothers produce a magazine titled Office Buildings. And there’s a guide to ongoing commercial construction in the city. The brothers have identified 750 projects that will be completed over the next three years. Some of their publications are populated by advertisements from high-end developers such as Donald Trump, and also brokers.

“The publications serve different niches,” Mr. Robbins said. They are mostly available by subscription, although their “Manhattan Condo Book” is a staple at most bookstores in New York.

The popularity of these publications has pushed Mr. Robbins to expand the business to 10 states. In particular, he’s promoting the newspaper, The Cooperator, currently published in New York and New Jersey.

This means Mr. Robbins is on the road several days each month, while his brother attends to the administration and management of the home office. This also means that a special concern of Henry’s is developing relationships with brokers, owners, and developers, and other sources of information, especially in the ever changing New York market. Mr. Robbins estimates that there are 80,000 condo units in Manhattan, and some 300,000 co-op units.

He provides regular updates to subscribers of his publications.

“You have to be always alert to what’s going on,” Mr. Robbins said. “This isn’t a business for anyone who’s slothful.”

Sloth certainly doesn’t seem to be one of Mr. Robbins’s characteristics.

“I’m very tenacious and very persistent,” he said. “I’m also very patient.”

Patience is something he most certainly inherited from his late father, Robert, who was a court stenographer. After retiring, the elder Mr. Robbins went to work for his sons as a collections agent.

Are bad debts a problem in the industry?

“Every business has its hiccups,” Mr. Robbins said, smartly skirting around specifics. “Our reputation is very solid, and we expect people who deal with us to also honor their word.”

His 86-year-old mother, Mary, works for her sons, too. She keeps the books.

“If you can’t trust your own mother, who can you trust?” Mr. Robbins said.