Publisher: Harper & Row (1971-1990); HarperCollins (1990-)
Years: 1971-early 1990s
ISBN format: 0-06-430xxx-c
Icon Editions was the pet publishing project of Cass Canfield, Jr., who started the series as "a mini-department within the general book division" at Harper & Row in 1971, according to a PW retrospective from 1983. The series primarily published art books and operated as more of a university press-style venture than most trade series.
As far as I can tell, the series designation IN- was used up to at least the late 1980s. It's difficult to discern, but it seems like after the merger of Harper & Row with Collins (to form HarperCollins) was completed in 1990 the series became a proper imprint, IconEditions. Around that time it appears as though the sequential numbering scheme was only loosely followed, with some titles being assigned non-sequential ISBNs. I've decided to take the series up to IN-200 (right around the year 1991) and stop there, given how uncertain it appears that the series existed as such into the mid-1990s. It's certainly possible to extend the sequence out a bit more through the ISBNs.
Harper & Row: Trade Scholarship, PW, Aug. 19, 1983, at 30 (profiling Cass Canfield, Jr., and his pet series, Icon Editions, then-extant for about 10 years and operating more as a university press outfit than most trade series).