I'm not entirely sure as I haven't read this novel specifically. But generally, 'isekai' often involves being transported to another world. 'Onsen' is a hot spring. So it might be about a protagonist who is reborn in an onsen in another world and their adventures or experiences there.
You can try looking for it in local bookstores that specialize in Japanese novels or manga - related books. Sometimes they carry unique and hard - to - find titles.
For me, 'That Time I Got Reincarnated as a Slime' stands out. The world-building and plot development are really engaging. Also, the characters have a lot of depth.
Well, it's a bit hard to pinpoint exactly which was the very first one. But some early examples could include 'El-Hazard: The Magnificent World', which had elements of the isekai genre.
Isekai Quartet indeed has a manga. It offers a different perspective and sometimes delves deeper into the story compared to the anime. The manga can be a great companion to the animated version for fans wanting more of the Isekai Quartet world.
I'd recommend checking out online manga libraries such as Comixology or Crunchyroll Manga. They often have a good selection of isekai titles. Also, some local libraries might have physical or digital copies you can access.