Irex, Cool-er, Asus.

Not exactly household names. But these companies are among the legion of devicemakers and others that are trying to get a foothold in the nascent electronic-book market.

The buzz around Amazon’s Kindle electronic reader has sparked growing interest in digital books, and the universe of players in this space is growing almost weekly.

But the still modest revenue associated with e-books raises questions about how many companies the space can support—and whether it’s headed for an eventual shakeout.

Amazon staked out an early lead with electronic books, betting that consumers will gravitate toward reading on digital devices. But as the list of competitors to the Kindle expands, Amazon may have a harder time keeping control of electronic-reader pricing, format, and content—the pillars of its digital strategy to date.

Among recent developments in the market: bookseller Barnes & Noble has formed partnerships with forthcoming electronic readers from Plastic Logic and Irex; Sony has announced a new lineup of e-readers; and Google is teaming up with various devices to distribute digital titles from its giant book-scanning project.

Amazon and its expanding list of rivals are betting that electronic books are the wave of the future and that people will eventually move in larger numbers toward reading on digital devices.

Electronic-book sales have been growing at a rapid clip, from an estimated $20 million in 2003 to $113 million in 2008, according to the Association of American Publishers.

But e-book sales are still relatively small compared with the rest of the book industry—making up less than 1 percent of the $24.3 billion in total book sales last year.

“I do think that lots of folks are intrigued by these devices and platforms, and there are conveniences and advantages that come with reading in digital formats,” said Ed McCoyd, who tracks digital issues for the AAP. But, he added, “e-books still make up only a small portion of overall industry revenues.”

Amazon.com does not reveal sales numbers for Kindle devices or electronic books, so it’s hard to get a handle on how the business is performing for the e-commerce giant (the AAP data includes e-books sold via Amazon but doesn’t break them out).

But Amazon is widely considered to have the lead in the market, and rivals are trying to challenge the Kindle on a range of fronts.