The publishing industry enjoyed a strong year in 2005, with increases in both revenues and the number of books sold. But projections for long-term growth remains limited because people increasingly don't read, according to a study released Friday.

Over the past few years, the number of books published has soared even as sales fell. That changed in 2005. A recent report by statisticians R.R. Bowker projected that the number of books released actually dropped last year, to 172,000.

The Book Industry Study Group, a nonprofit trade association, announced Friday that the number of books sold in the United States increased by 3.8 percent, to 3.1 billion.

"I think it was overall an impressive year," said Al Greco, a senior researcher for the Institute for Publishing Research who cited increased sales for religious books, children's titles and educational works.

In actual dollars, net revenue increased 5.9 percent to just under $34.6 billion. But the study group expects a slow market for the rest of the decade, citing an emerging generation that doesn't read as much as the previous one.

"The average amount of time and money spent by the general population on books has been declining and we don't see anything that's going to change that," said Bob Wharton, also a senior researcher at the Institute for Public Research, based in Bergenfield, N.J.

"Given that the older population tends to be heavy readers, as that population ages further, we're probably going to see a further decline overall in how much time is spent on books as opposed to other media."

The 2005 statistics were weighted to include sales for smaller and middle-sized publishers that had been previously left out because of a lack of reliable data. The study group last year issued "Under the Radar," a report which estimated that such companies brought in around $11 billion in annual revenues.