This manga was originally published two years ago, and the artist decided to make it freely available on Pixiv recently. Not knowing Japanese, my understanding of the plot is that Marisa wakes up in a world where she never left her parents' house and has grown up an ordinary girl, without learning magic or meeting Reimu and Alice. However, she refuses to accept this and attempts to reestablish her other life. Spoiler'd for anyone who cares.
Wow that sounds interesting. Great on the artist to offer it freely too.
I actually don't think it's that uncommon for artists to do that. I've seen it done more than a few times. I mean, for one thing, if it's not in print anymore they aren't making money from it. For another, I think it's pretty easy to do, assuming they haven't moved or deleted the files. Plus, a lot of artists are already aware that their print works are often scanned and uploaded online in the west for people who don't have the means to buy it themselves. So other than not wanting to go to the trouble, there's not really any reason for an artist not to do it.
Of course, that's all speculation, so maybe I'm completely off the mark.
EDIT: Upon rereading this, I want to clarify that I don't mean to imply it's not admirable for an artist to upload their old works. Just that it's more common than one might think.
Well thanks for EXPLAINING THE ENTIRE PLOT of the book on the very first page. I understand, using spoilers beacons is sooooo hard...
"Plot" and "premise" are two different things. If I told you Alien was a movie where a space ship's crew finds an alien being that pops out of people's chests and starts killing people, that's not "spoiling the plot", it's "telling you the very basics of what sort of movie this is".