BRENTWOOD, Tenn. -- It happens every time Jeff Green and his wife go out to listen to music. They hear an unfamiliar song, exchange a look, then pull out notepads and pens.

"Some people collect cuckoo clocks," Green said. "Some people collect movie posters. We collect songs, and then classify them."

Green and Lauren Virshup publish "The Green Book of Songs by Subject." It's a more than 1,500-page monument to obsession that's fun for music fans to breeze through.

We're talking more than 35,000 songs divided into more than 1,800 categories. Love songs alone take up 147 pages, sorted into subgroups such as "Crushes" and "Don't Want to Break Up."

The book -- used by disc jockeys, libraries, public speakers and basic popular music geeks -- has sold more than 20,000 copies since the first edition was published in 1982.

Envisioned as a book for music industry professionals, "The Green Book" has become popular in other fields.

"We sold a copy to the San Diego Padres, National Public Radio and ABC News," Green said. "Public libraries and schools have been good customers. to reach their congregations. Music therapists have bought it, and mobile DJs who do parties and weddings. Many teachers find it useful."

Green would like to bring the database to the Internet for a subscription fee, or a CD-ROM, if it can be protected from piracy.

"There are thousands of albums released every year, so we never really will catch up," he said. "In a sense, I don't have to worry ... if it never gets finished. As long as we keep doing it, it will stay vital."

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On the Net:

"The Green Book of Songs by Subject" Web site: http://www.greenbookofsongs.com/