this image belongs to a doujin named: Kakusei wa Migiwa ni Madou ～Chi no Maki～ (you can find it here: https://e-hentai.org/g/214859/ad2f689835/) and it is surprisignly 110 pages long... it's also tells the story on how sanae came to gensokyo too... and also, is a sequel to this other doujinshi named: Kakusei wa Migiwa ni Madou ～Ten no Maki～, actually, i think there's a triology of books. the first is only 44 pages long, while the other two (this including) are a bit more longer... (110 pages for this one, and a 160 pages for the last one).