Amazon's Kindle 3G digital reader is getting cheaper. Inc (NASDAQ: AMZN) will partner with AT&T (NYSE: T) to sell to its popular device for $139 with free 3G service for the life of the product.

There's a catch, however. AT&T's sponsorship of the low-priced Kindle 3G means the company will get to place built-in ads on the device's screen-savers. So buyers of the AT&T-sponsored Amazon Kindle 3G digital reader will get a discount in exchange for getting AT&T ads on a daily basis.

Amazon's Kindle 3G digital reader typically costs $189. It allows buyers to purchase or download free electronic books and other digital media information without having to pay a monthly subscriber fee. The Kindle 3G operates off a network, so if buyers can handle the advertisements that pop up from AT&T, they can now get convenience of free 3G mobile networking for the same price of a Wi-Fi only Kindle.

Kindle 3G customers read 20 percent more books, and take advantage of twice as many special offers, said Jeff Bezos, Amazon's founder and CEO, in a statement. A big thank you to AT&T for helping to make the new $139 price possible.

Amazon introduced in April built-in advertising on a different version of its Kindle digital reader, partnering with companies including General Motors, Proctor & Gamble, Olay, and Visa. The Kindle is the bestselling digital reader. It competes with Barnes and Noble's Nook, Apple's iPad and other products. Amazon is also the world's biggest retailer of ebooks.

Last year Amazon dropped the price of its Kindle 2 3G from $259 to $189. But the Kindle 3G with Special Offers has been selling for $164.

A Wi-Fi-only version of the Kindle with Special Offers sells for $114.

Customers purchased the new $139 Kindle 3G through AT&T's partnership can have free 3G mobile service for the life of the device.

Kindle 3G is by far the fastest-growing connected device on the AT&T network, AT&T Mobility CEO Ralph de la Vega said in a statement. We consistently hear from customers how much they enjoy the freedom of not being tied to a Wi-Fi hotspot and instead being able to download and read new books anytime, anywhere...