Meet the John C. Dvorak of cyberspace. He's Adam C. Engst, the 26-year-old editor of a free electronic newsletter about "the world of the Macintosh and electronic communication" called TidBITS, one of the most popular publications on the Net. TidBITS's 100,000 readers depend on Engst for the latest news about Apple, Inc., Macintosh hardware upgrades and insider information from industry sources. Engst, who just released issue No. 237 of the weekly newsletter, is the author of three books on the Internet -and he's quick on the draw "My deadline is about an hour before it goes out Monday night." For more info send e-mail to email@example.com; back issues available at ftp.tidbits.com.