I’m looking for the Pokémon Card Game Grand Party 1999-2000 Challenge Road ‘99 SUMMER Official Regulation Book (ポケモンカードゲームグランパーティ1999-2000チャレンジロード’99SUMMER公式レギュレーションBOOK). It was included with the June 1999 issue of CoroCoro Comic (#254). Here’s a link to a sold listing for reference. That one sold for ¥810, but I don’t see it pop up too often, so I’ll offer $20.
If you have this, even if you don’t think you want to sell, please let me know.