According to Official Twitter Spokeswoman:
Twitter is testing the feature with a small percentage of users. It was designed to “make it easier for users to engage with each other” on the microblogging service."
Link to Original Source
Catch up on stories from the past week (and beyond) at the Slashdot story archive
The best book on programming for the layman is "Alice in Wonderland"; but that's because it's the best book on anything for the layman.