Here's a sure sign that the nascent e-reader market is starting to mature: Key players are suing each other.

Spring Design today filed a lawsuit claiming that Barnes & Noble illegally used Spring's technology in the book retailing giant's recently unveiled wireless periodical device, the Nook.

The smaller firm on Oct. 19 unwrapped details of Alex, its upcoming, Android-powered, dual-screen e-reader. Now it's charging in a lawsuit that Barnes & Noble (NYSE: BKS) stole trade secrets and violated a non-disclosure agreement between the two companies.

Cupertino Calif.-based Spring Design charges that Barnes & Noble copied some of Alex's features for the Nook e-reader, which it showed off Oct. 21 and is expected to begin selling later this month. Like the Alex, the Nook also features a dual-screen and runs on Google's (NASDAQ: GOOG) open source mobile platform, Android.

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