NEW YORK (AP) _ Plenum Publishing Corp., a specialized publisher of scientific books and journals, said Monday it offered $50 a share to acquire the Arthur D. Little, Inc. consulting firm.

Little, which is based in Cambridge, Mass., has 2.6 million shares outstanding, giving the offer an indicated value of $130 million.

In a letter to Little chairman John F. Magee, Plenum said it wanted to acquire 70 percent of the shares held by the Memorial Drive Trust, the company's retirement fund, as well as the remaining Little shares held publicly in a ''non-disruptive, friendly manner.''

Plenum also indicated it was prepared to increase the offer if warranted by a review of the firm's finances.

Martin Tash, Plenum's chairman and chief executive, said he had attempted unsuccessfully to meet with Little executives over the past several weeks to present his offer.

Michael O'Hara, a spokesman for Little, said the company's board ''will meet in the near future to consider Plenum's offer.'' He had no further comment.

Founded in 1886, Little is a technical and management consulting firm with offices worldwide. The company reported 1986 net income of $6.5 million on revenue of $248 million, a spokeswoman said. Little has about 2,600 employees.

Little said Monday its revenue for the second quarter ended July 3 was $69.3 million, up 9 percent from a year earlier, while its net income rose by 10 percent to $1.8 million.

Little shares soared $11.75 to close at $46 a share in national over-the- counter trading. Plenum rose 75 cents to close at $27.75 a share.

Under Plenum's proposal, Little would operate as an independent subsidiary with its current management and workforce intact. Tash also said Plenum would continue contributing to the Memorial Drive trust.

Plenum offered to purchase the shares held by the Memorial Drive Trust for $50 cash apiece, or alternatively for $40 in cash and $10 in convertible debentures. Tash said the remaining outstanding shares would be purchased under the same terms in a subsequent merger.

Plenum reported 1986 net income of $12.2 million on revenue of $38 million.