The Macmillan Paperbacks were announced late 1959, a few months before Crowell-Collier started on its year-long plan to purchase the Macmillan Co. The merger of the two companies, and the operation of Macmillan Co. as a subsidiary of Crowell-Collier, was completed on December 30, 1960, by which a good number of Macmillan Paperbacks had already been published. It was announced mid-1961 that a select number of Macmillan Paperbacks would be incorporated into Crowell-Collier's ambitious 50-books-a-month Collier Books imprint, although the Macmillan Paperbacks proper continued as a separate line.
In 1965, Crowell-Collier officially changed its name to Crowell Collier & Macmillan, Inc. (the Crowell-Collier branding would be dropped entirely in favor of the Macmillan name by the early 1970s.) Starting in 1965, Macmillan Paperbacks continued as a separate series but dropped the regular MP- series prefix, adopting instead Collier's 5-digit numbering, which seems sequential but shared among all the publisher's books; as a result, my list stops in 1964 with the last MP-designated title, MP-184.
Initially, I had a gap in my numbering of this series from MP-15 through MP-21. PW, in a 1959 article, mentioned the Macmillan Poets series, which had started that year, as continuing as a separate series from the Paperbacks. This may have been planned, but I don't think that's how it ended up. From my research in PW, there were 6 titles published in 1959 in the Macmillan Poets series. I think those six titles were considered (or actually re-released as) MP-15 to MP-20. That would explain the gap.
The next gap was from MP-35 through MP-37. As it turns out, MP-35 is W.S. Merwin's Drunk in the Furnace (1960), the next title in the Macmillan Poets series, according to PW. It seems like at some point the Poets series was either subsumed into the new Macmillan Paperback series or actually was part of the Macmillan Paperbacks all along. PW, Sept. 25, 1961, at 52, has 4 new Macmillan Poets titles being published Oct. 30, 1960: Dilys Laing's "Poems from a Cage" (WorldCat has this as MP-36), David Schubert's "Initial A" (WorldCat has this as MP-37), Francis Golffing's "Selected Poems" (found a copy on AbeBooks that shows MP-62 on the spine), and Harold Witt's "Beasts in Clothes" (WorldCat has this as MP-63 and the title kept getting delayed according to the forthcoming releases in PW).
As mentioned above, Macmillan Paperbacks started bearing a different kind of series indicator in 1965.
The Abolition of Man (C.S. Lewis, Sept. 1965) -- #06623
The Irreversible Decision 1939-1950 (Robert C. Batchelder, Sept. 1965) -- #08364
Creation and Fall / Temptation (Dietrich Bonhoeffer, Sept. 1965) -- #08389
Between Man and Man (Martin Buber, Fall 1965) -- #08420
Out of the Silent Planet (C.S. Lewis, Sept. 1965) -- #08688
Perelandra (C.S. Lewis, Sept. 1965) -- #08690
That Hideous Strength (C.S. Lewis, Sept. 1965) -- #08692
Garlands for Christmas: A Selection (Chad Walsh, Sept. 1965) -- #200 / 08969
The New Creation as Metropolis (Gibson Winter, Fall 1965) -- #09014
The Secular Meaning of the Gospel (Paul Van Buren, 1966) -- #08976
The Standard Book Number (SBN) wasn't implemented until 1966, and the ISBN followed several years later. Not sure what Macmillan (or Collier?) was doing with these numbers.
Macmillan Paperbacks to Start Next Spring, PW, Nov. 16, 1959, at 34-35 (indicates there were 13 initial titles tentatively scheduled for publication in March 1960).
Crowell-Collier Buys Bloc of Macmillan Co. Stock, PW, Nov. 16, 1959, at 36 (announcing that Crowell-Collier purchased 50,000 shares in Macmillan Co., just less than 15% of its outstanding stock -- the first move in a planned takeover).
"Barron's" Sees Investment Gains in Book Publishing, PW, Feb. 15, 1960, at 137-38 (mentioning that Crowell-Collier now owns about 30% of Macmillan).
Ad, PW, June 13, 1960, at 5 (listing MP-5 as published & MP-22 through MP-34 as "coming September 12 [1960]).
Book Publishing Stock Prices: A Monthly Report, PW, Aug. 1, 1960, at 31 (reporting that Crowell-Collier has now acquired at least 51% of Macmillan stock).
Crowell-Collier, Macmillan Agree on Merger Terms, PW, Oct. 24, 1960, 28-29 (announcing the merger, terms of which included Macmillan continuing to operate under its name as a subsidiary of Crowell-Collier).
Crowell-Collier, Macmillan Stockholders Vote Merger, PW, Dec. 26, 1960, at 33-34 (reporting official merger of Crowell-Collier and Macmillan, effective date December 30, 1960).
Ad, PW, Feb. 6, 1961, at 38 (listing Macmillan Paperback titles to be published March 13, 1961, including titles from MP-38 to MP-55).
Crowell-Collier Paperbacks Start in Fall, 50 a Month, PW, June 12, 1961, at 103-04:
The books in the Macmillan Paperback line "which lend themselves to broader distribution" will be absorbed into the Collier Books program as the books begin to go back to press, starting early in 1962. The Macmillan Paperbacks will be reset, with new covers and fitted into the Collier Books price range. Among the Macmillan titles to be eventually acquired for Collier Books in this fashion will be "Gone With the Wind," the volumes of Sean O'Casey's autobiography, "The Golden Bough" and works by J. B. Phillips, Arthur Koestler, Walter Lippmann, and many more.
Name Change Proposed: Crowell Collier & Macmillan, PW, Mar. 22, 1965, at 52.