When the Pokémon cartoon was at the height of its popularity in the States, a bunch of books were published as well that more or less follow the series. Thundershock in Pummelo Stadium follows Ash on the Orange Islands, a series I'm not all the familiar with. According to Amazon.com, it is the 16th book in the Pokémon chapter books adapted by Tracey West.
Ash has one more badge to win before he can join the Orange Islands league and battle in the league wide competition. Thundershock covers the last badge and his competition in the Winner's Cup on Pummelo island.
There are some side plots involving a quest for an antidote to Vileplume stun spores and some brief interference from Team Rocket. Ultimately though the book focuses on the full battle (all six pokémon taking turns).
If you're not a regular player of the games or fan of the dubbed series, this book can be skipped. If you are a fan of either the series or the games, give the book a go. It's short and entertaining for what it is.