Amazon.com is still the leader in eBook sales, and its closest competitor isn't Apple—it's Barnes & Noble.

As the publishing industry on May 26 wrapped up four days of digital talk at its annual national convention, Amazon.com’s Kindle was seen as the clear, if not dominant, player in the growing eReader market; Barnes & Noble’s Nook was considered a pleasant surprise and Apple’s iPad an underachiever.

“They had a respectable launch, but we think Apple can do better,” Penguin Group (USA) CEO David Shanks said this week during BookExpo America at the Jacob Javits Center in New York. “They still haven’t moved their eBooks into their iTunes store, and they can have a much better search capability in their iBookstore.”

“The iPad offers so many audio visual applications that reading is not given as much priority as it is in dedicated [reading-only] devices like the Nook and Kindle,” says literary agent Richard Curtis.

More than 20 million iPads and iPad 2s have been sold over the past year, and Apple’s iBookstore is also available on more than 160 million additional devices through the iPhone and iPod. But publishers and agents say Apple is not yet the balance to Amazon.com for which they had hoped. They estimate that Apple sales are around 10 percent of the eBook market, far behind the believed 60 percent to 65 percent for Amazon.

Publishers and agents say eBooks are at least 15 percent to 20 percent of overall sales, more than double from just a year ago.

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Apple spokesman Jason Roth declined to comment on any specific criticisms, but he did say that the iBookstore had more than 150,000 titles—an Amazon spokesman says the Kindle store has more than 950,000—and that more than 100 million books had been downloaded worldwide through the iBookstore. He would not say how many were downloads of free books. Selections at the iBookstore were greatly improved this year when Random House Inc., publisher of Stieg Larsson and John Grisham, among others, agreed to sell through Apple after resolving differences over pricing.

Brian Murray, CEO of HarperCollins Publishers, said iBookstore sales were “a little smaller than expected,” but he praised the iPad as a multimedia breakthrough that enabled publishers to sell electronic picture books and “enhanced” eBooks that include video and sound.

“There are certainly areas for improvement, as there are with every book retailer and device,” he said. “But the promise of having another platform where books can be discovered is still true today. The potential is enormous.”

A strong No. 2 to Amazon has emerged, but it’s Barnes & Noble, which launched the Nook late in 2009 to skepticism about everything from the name “Nook” to the design.