To "Talk" to any of the people on this game you have to advance the relationship with gifts, specially for quest and even simple sex scenes; so, to start most quests, you have to start by gifting them, usually 3 of the most expensive food from the beach vendor, and they will talk. I tested with the chocolates and they arent very good at raising relationship with villagers, But I used a Mtranslator with a jap ver. so it may be changed.
To get the porn book you have two options:
Fastest(relatively): max out relationship with the kid that likes the white haired girl and its always close to her or hangs on the outside of the bar near the grandpa, cant remember his name but hes not the blonde kid. His quest will be to bring him 1 of the most expensive food from the beach vendor. Make sure to do this before only having 1 time before night, otherwise the quest might bug out and you will have to force-quit.
Slowest: Complete grandma's questline, after gifting, talking and completing the whole grandma(and her daughter the shop owner) quest and I think you also have to put the tofu on the story related location(one screen below your house), then you will get the shop discount and the ability to buy some more things, between those is the porn book at 100G, the other one is for another quest.
I recommend either grinding stats for the first few days so you can start the new game+ with 1 extra action and some points, or just cheat the money and points, its way better. Because this game is grindy as heck, even worse how the best money-making activity I found was swimming, but it always consumes the whole time slot, no matter how many day actions you have.