LOS ANGELES, July 30 (UPI) -- Online retailer Amazon.com has introduced what it says is the least expensive electronic book-reader available.

The device, called Kindle, retails for $139, and goes on sale next month. The cheapest Kindle operates only in an environment that has wi-fi, but a $189 version is equipped with 3G, the same communications platform used by many cellphones.

Last month, the cheapest Kindle e-reader listed for $259. The new reader has a sharp screen and is lighter than a previous version, the Los Angeles Times reported Friday.

Amazon.com said its electronic books sold better than conventional hardcover books last month