Sites

https://fogu.com/rk5/ : Incredible fansite which gives in depth information on every facet of the game lore and gameplay mechanics in english. It even has forums for the game that saw activity in 2025.

https://www.marv.jp/special/game/ds/kawanushi/ : Official website, all in japanese. There are newsletters about the game’s production and release from the time, for example a newsletter on release day from the staff, don’t be scared of the very retro interface this is a treasure trove. There are very short exclusive comics not found anywhere else. Graphics not found anywhere else as well.

Books

There is an official strategy book with lore and production info, but mostly game mechanics, called 川のぬし釣りこもれびの谷せせらぎの詩完全攻略ガイド. It's 143 pages and you can read its release announcement here. It's still rather easy to find being sold online.

Fami2comic (ファミ2コミック) ran a River King Mystic Valley manga by Igusa Matsuyama from july 2007 to september 2007 in their magazine issues. You can read its announcement here. To read it yourself, I recommend: https://archive.org/details/fami2comic200707/078-079.jpg 79 to 94 for chapter one, from the july issue. https://archive.org/details/fami2comic200708/118-119.jpg 119 to 134 for chapter two, from the august issue. https://archive.org/details/fami2comic200709/128-129.jpg 129 to 136 for chapter three, from the september issue. And https://randomhoohaas.flyingomelette.com/ai/scans/fami2comic.html#riverking for overall fami2comic info

Its first two issues were however released in its own small book. This book is labeled as book no. 01, but as far as I know there was never another one, since there was only one more chapter afterward. I couldn't find it stored anywhere online, but I do have a physical copy I might get the bravery to scan one day, it seems to have a few exclusive things not included in the other Fami2comic issues. It's 35 pages, and very hard to find sold anywhere.

Misc

This site compiles all releases’ case art and some assets. https://gamesdb.launchbox-app.com/games/images/14727-river-king-mystic-valley

The soundtrack can be found here. https://downloads.khinsider.com/game-soundtracks/album/river-king-mystic-valley-ds-gamerip-2007 To my knowledge it was never released officially in any form.

A video of the original japanese trailer can be found here. https://archive.org/details/nushiCM

There is a magazine ad in english for the game, from Nintendo Power issue #228 from May 2008, that measures approximately 8x10.5 inches. The second one is from a Famitsu magazine.

川のぬし釣り外伝～魔法の枕と大っきな魚～ / River King Fishing Gaiden -Magic Pillow and Big Fish : There was a spinoff released in 2008 as a mobile/android game. Japanese exclusive. It seems to be lost to time however…  https://japan.zdnet.com/release/10363977/ 1 https://www.dreamnews.jp/press/0000001951/ 2 https://www.inside-games.jp/article/2008/01/29/26669.html 3 It even got a second installment: 川のぬし釣り外伝２～夏祭りと肝試し～ / River King Fishing Gaiden 2: Summer Festival and Test of Courage https://www.dreamnews.jp/press/0000002528