When I started college my computer was an Atari 400 I had purchased with money from my after-school job programming commercial software. I had replaced the membrane keyboard with a third party “real” keyboard and had the Action! cartridge. I used the editor primarily to write papers for class printed on my dot-matrix electric arc printer (it produced the dots by scorching the paper with a tiny carbon electrode thus needing no ink, though the print was brown and lower contrast).
Sadly I sold the 400 to someone I knew for their child and loaned the Action! cartridge to a co-worker who moved across the country and we lost touch. I’ve been searching eBay for years for a copy of the manual as well - at one time I had an ambition to create a version for iPad or Windows.