Publisher: Grove Press
Years: 1949-
Related Series:
ISBN format: 0-8021-xxxx-c (not sequential); 0-394-55xxx-c (Random House for distribution; not sequential)
Breaking all the rules to include these, as they're mostly not paperbacks.
Notes:
The first three titles were published by the original Grove publishers from 1949-1950.
Barney Rosset purchased Grove in 1951 and commenced publishing hardcover titles.
As far as I can tell, GP- numbers were not used during the early years on the books themselves, though it's possible they were used internally, as for inventory purposes.
GP- numbers for these early titles come from UNC's Grove Press Imprint collection. The catalog notes often indicate that the copies in the collection have spine labels with GP- numbers.
Grove acted as a distributor for smaller presses early on, and some of these titles in the UNC collection also have GP- numbers, despite the physical items themselves not being marked as Grove publications in any way.
Examples include GP-16 & GP-17, from Banyan Press. GP-18 is a book originally published by the Cresset Press in England that a publisher/importer called the Chanticleer Press, issued in the U.S. as part of its Cresset Library in late 1949 (see ad, PW, Sept. 3, 1949, at 883). It appears that Grove also later reissued other Cresset Library titles as Grove clothbound titles (e.g., GP-28, GP-173). Also, James Broughton's Musical Chairs: A Songbook for Anxious Children (GP-72) appears to have been originally published by The Centaur Press in 1950 and not issued as a separate Grove publication.
GP- numbers begin to appear in Grove Press ads before being printed on the dust jackets (see, e.g, PW, Jan. 28, 1957, at [50]—though the GP- numbers appear to be one number lower than what's in the UNC collection for these titles—and PW, Jan. 23, 1961, at [61]); the GP- numbers begin to appear on the spine and front inside flaps, near the price, around late 1964, possibly with the fall titles.
Some Grove clothbound titles were issued with a GT- number:
GT-308: Popkin, ed., The New British Drama: Six Plays, in 1964 at $8.50, labeled on the spine as a "Text Edition." This title was later issued as GT-614 under the title Modern British Drama: Six Plays in 1969 in a paperbound edition at $5.95.
GT-325: Clurman, ed., Seven Plays of the Modern Theater, originally GP-260 at $8.50 in 1962. This version was labeled as a "Text Edition" in 1964 at $4.50. The title was also later issued in a paperbound edition as GT-422, also labeled as a "Text Edition," in 1967 at $3.95.
A few GP- titles bound full volumes of Evergreen Review issues together, according to the 1965 PTLA (note that spines of Vols. I-II may be in an image from an auction lot at an Irish auction site from 2010):
GP-157: Vol. I (1959, $7.50)
GP-179: Vol. II (1959, $7.50)
GP-247: Vol. III (1960, $3.50)
GP-334: Vol. V (1965, $7.50)
Grove published, at Henry Miller's behest, a Russian translation of Tropic of Cancer as GP-333, limited to 200 copies. There's a listing with images and more information at a Russian-focused rare books seller's website and a listing with photo of a sold fine copy at Between the Covers.
Subsequent printings are indicated by lowercase Roman numerals on the rear jacket.