Complete her route and also donate 50 books to the library (buy cheap novels from the city vendor every Sunday). The library stays open, she publishes her novel, and the final scene shows you two reading together on the farm porch.
Beginner's guide for Summer Clover without (too many) spoilers Summer Clover Walkthrough
: Certain "???" actions in locations like the hallway can trigger rare item drops, such as the sisters' undergarments, though these increase player stress when used. Complete her route and also donate 50 books