More than 50 million U.S. adults now read electronic books, according to media and publishing research firm Simba Intelligence, a unit of Market Research Group LLC. But those consumers did not spend more on average for digital reading material in 2012 than in 2011, the firm says, without providing exact spending figures.

Simba Intelligence has been tracking the habits of e-book readers since 2009. It published its fifth annual “Trade E-Book Publishing” report this week. Simba did not give a previous figure for the number of e-book readers, but said 2012 was first time e-book readers passed the 50 million mark.

“In the last edition, we noted the gap between e-book users and e-book buyers grew wider than expected, and that was the warning sign a lot of people ignored in 2011,” says Michael Norris, a senior analyst in Simba Information’s consumer media and technology division and an author of the report. “Not only did the gap grow even wider in 2012, but the average amount of money spent by a given e-book buyer didn’t rise, which makes what a lot of people think was a simple slowdown in adoption a lot more complex.”

The report proposes a mix of reasons for last year’s flat e-book spending. They include the “distracting” effect of tablets, which more consumers now own but do not necessarily use for reading e-books; consumers generally spending less overall in 2012; and the closing of bookstores, notably the Borders Books retail chain, which went bankrupt in 2011 and shuttered its 633 stores. Such closures may have prevented some e-book readers from browsing paper copies before buying digital versions, Simba Intelligence says. It also says that 20% of e-book readers bought a print book from a retail chain bookstore in the last three months and 13% bought one from an independent bookstore.

Simba adds that tablets capable of multiple functions are steadily replacing devices only for e-reading. The report uses as evidence decreased sales of Barnes & Noble Inc.’s Nook e-reader during the 2012 holidays. Despite releasing two new devices last year, Nook HD and Nook HD+, sales of Nook tablets, e-readers, digital content and accessories decreased year over year 12.6% in the nine-week holiday period, the retailer says. And although multi-function tablets support reading e-books, their owners are not more likely to purchase that type of content than are other consumers, the research firm says. About 63% of smartphone owners, 48% of iPad owners and 40% of non-iPad tablet owners do not consume e-books, Simba says.

Simba Intelligence cautions retailers that simply making content available digitally isn’t enough to drive consumers to purchase. “The e-book industry is much more nuanced than most people think and getting a person to value and engage with your content isn’t always going to be an easy sell,” Norris says.