Norman Lewis, who died September 8 at 93, was one of the most obscure best-selling authors of the 20th century.

He was author of such multimillion-selling titles as “30 Days to a More Powerful Vocabulary” and “Word Power Made Easy,” as well as editor of a revised dictionary-style edition of Roget’s Thesaurus that sold more than 5 million copies.

A former magazine writer who eventually became an English professor, Lewis eschewed the high life and lived in a modest home near Rio Hondo College, in Whittier, Calif.

Lewis was born in Brooklyn and was raised by a sister after being left an orphan at age 5. He published his first article at age 11, a prize-winning book report that was reprinted in the pages of the New York Tribune.

A slow study who enrolled late, he took twice the normal time to get through City College (he eventually got a master’s degree in English from Columbia). He supported himself by writing articles for magazines. His first was a quiz on the varieties of mania, which ran in Leisure in 1939. It was the beginning of a 15-year career writing for magazines.

Lewis’s first book was a sixth-grade textbook, “Journeys Through Wordland” (1941). He followed this with the more successful “RSVP” (reading, spelling, vocabulary, and pronunciation), which was eventually expanded into six volumes for college instruction.

In the early 1940s, he was approached by the editor and publisher Wilfred Funk – of the Funk & Wagnalls Co. – to be his partner in writing what eventually became “30 Days to a More Powerful Vocabulary.”

Funk’s name appeared as author, but Lewis contended that he did most of the work. “I realized I was being had, but I thought it was a good way to get started,” Lewis told the Los Angeles Times in 1978. When the book appeared in 1942, Funk advertised it heavily and, Lewis said, “it sold like mad.”

“Thirty Days” is still in print, in multiple editions, and contains a handy index of its high-powered words, from abortive and abstemious to unicycle and wanton. At least 6 million people have fallen for its promise to “make words your slaves.”

“Give us 15 minutes a day, and we will guarantee that at the end of a month, your conversation and your life will have a new and deeper meaning for you,” the book’s introduction promises. That is, if you don’t end up feeling too much weltschmerz, the final word in the index.

Lewis became a nationally recognized expert on words and published dozens more books that were essentially expanded magazine pieces, with titles like “Mind Over Grammar” and “Instant Word Power.”

In 1952, he took a leave of absence from City College, where he was then teaching English, to compile a thesaurus that turned out to be one of his few unsuccessful books. He was subsequently hired to revise Roget’s Thesaurus in an alphabetical format, which proved to be another giant hit, first published in 1961 and still in print.

Lewis moved to California in 1964 to become part of the founding faculty of Rio Hondo College. He became a beloved teacher and faculty eminence, but did not often talk about his books, preferring to concentrate on matters pedagogical.

Lewis lived modestly and drove an old Toyota. “He would disappear every summer to write a book,” a retired spokesman for the college, Ted Snyder, said. He refused to do any kind of publicity for his books, and he retired in 1995.

In his spare time, Lewis read novels — one a day when classes were in session, twice that otherwise. For tips on emulating his trencherman’s appetite for literature, consult his book “How To Read Better and Faster,” first published in 1944 and in print for decades, now only available used, ubiquitously.

So what did this quintessential word man do for relaxation? “His hobby was crossword puzzles,” Mr. Snyder said.