By Liana B. Baker

NEW YORK (Reuters) - Book retailer Barnes & Noble Inc debuted an e-reader on Thursday with a screen that can glow in the dark, a move aimed at improving its position against Amazon.com.

The 6-inch (15-cm) touch-screen device, which will hit stores in early May for $139, lands in a market where Apple Inc and Amazon are battling with Barnes & Noble for customers who want to read books and magazines on small portable devices. Weighing just under 7 ounces (200 grams), it is Barnes & Noble's lightest Nook yet and is aimed at people who want to read in the dark without a light.

To read in the dark with other e-readers that use electronic ink, consumers had to buy an external light that attached to the device. While Apple's iPads have backlit screens, Barnes & Noble said its technology creates a better nighttime reading experience than an LCD screen because it throws off less light and will not disturb a reader's partner who is trying to sleep.

While Barnes & Noble is first to market with a glowing e-reader, Amazon may be hot on its heels. The Internet retailer is working on releasing its own e-reader with a glowing screen, according to a report on April 6 from the blog TechCrunch. Amazon did not respond for comment on the report.

"If Amazon isn't already working on one, they will immediately start working on one now after today's announcement," Forrester analyst James McQuivey said.

Fighting deep-pocketed Amazon has proved to be an expensive proposition for Barnes & Noble, as it has poured hundreds of millions into developing the Nook. In January, the retailer forecast a steeper net loss than expected for the fiscal year, as it continues to spend on the technology.

"We are investing a lot but we feel good that those investments will pay off," Barnes & Noble Chief Executive William Lynch said in an interview on Thursday.

He added that the chain, the No. 2 seller of e-books behind Amazon, is increasing its market share faster than any other company. In February, he estimated the company captures 27 percent to 30 percent of the U.S. digital books market.

Barnes & Noble introduced its first version of the Nook in 2009, two years after Amazon's Kindle.

Lynch declined to comment on whether Barnes & Noble would change the prices of its e-books after theDepartment of Justice reached a settlement with three major book publishers that it says colluded withApple to push up e-book prices.

Amazon had said it plans to lower its e-book prices. Apple did not comment on Wednesday on a lawsuit filed against it and five booksellers alleging collusion.

"We'll come out with some points of view on that in the coming weeks," Lynch told Reuters.

Users can turn off the glowing feature on the screen to read outdoors as well, while the battery can last one month with the glow on without Wi-Fi. The $139 price on the new Nook is the same as that of KindleTouch without ads.

McQuivey, the Forrester analyst, said the device would appeal to Barnes & Noble customers, as well as consumers who have never bought an e-reader before. Attracting Kindle users will be a tougher proposition.

"To its core base, it says, 'we are still committed to this,' but if you owned a Kindle for years, there's no way you're going to move to Barnes & Noble at this point," McQuivey said.

Barnes & Noble shares fell 72 cents or 6.13 percent on Thursday to $11.05. Amazon shares rose $2.73 or 1.4 percent to $190.69 per share.

(Additional reporting by Phil Wahba; Editing by Dale Hudson)