NEW YORK, JULY 13 -- Plenum Publishing Co., a tiny publisher of technical journals and books with $12 million in profits last year, today offered to pay about $128 million to acquire Arthur D. Little Inc., one of the nation's leading technical consulting firms.

The offer was disclosed when Plenum released a copy of a letter that it sent today to John F. Magee, Little's chairman and chief executive, following what a Plenum executive said were several weeks of fruitless attempts to meet personally with Magee.

In the letter, Plenum Chairman Martin E. Tash offered $50 each for the Little shares held by the consulting firm's Memorial Drive Trust, a tax-exempt fund for employes of which Magee is the trustee. Plenum said that after a merger, it would offer the same terms to Little's remaining shareholders. Alternatively, Plenum said it would be willing to offer the trust $40 a share plus $10 in debentures convertible into Plenum stock.

The trust owns about 70 percent of Little's 2.56 million outstanding shares. The largest outside shareholder is Quest Advisory Corp., a New York mutual fund group, with about 6 percent. Sources close to Plenum contend that Quest favors the deal, but its fund manager could not be reached to confirm that.

A.D. Little stock has been trading over the counter at about $35. The stock closed today at $46, up $11.75. Plenum shares closed in over-the-counter trading at $27.75, up 75 cents.

The offer satisfies some mild curiosity in New York investment circles over how Plenum would use $50 million that it raised in April through an issue of subordinated debentures. The issue was underwritten by Bear, Stearns & Co., which subsequently proposed that Plenum use the proceeds to bid for A.D. Little, Tash said. In all, Plenum has a war chest of more than $90 million, including the securities proceeds and $40 million in internally generated cash. The company also has a credit line from Bank of New York.

For all that, Tash said, "we are proposing only the friendliest of deals. This is not a raid but a good business deal."

Magee, reached by phone in his Cambridge, Mass., office, said tersely to a reporter before hanging up: "We have received the letter and will respond to it in due course."

In a statement issued later, Little said its board will meet over the offer "in the near future."

The 101-year-old A.D. Little is one of the country's best-known multinational consulting firms, specializing in research and engineering. Last year, it earned $6.5 million on revenue of $248 million. The company today announced sharply higher second-quarter and first-half 1987 revenue and profits. In the second quarter ended July 3, it reported net income of $1.8 million, up 10 percent from 1986, and revenue of $69.3 million, up 9 percent.

Plenum is a 40-year-old New York publishing house whose most important asset appears to be an agreement with the Soviet Union giving Plenum the rights to execute and distribute English translations of a raft of Soviet technical and scientific journals