While I was searching images, I came across this.
The book is referred to as ポケットモンスター ゲームブック ("Pocket Monsters Game Book"). It appears to have been illustrated by Mato, the artist that drew the original chapters of the Pokemon Special manga (I found it on her site). You can see this clearly from the fact that Satoshi looks more like Special Red than he does in the anime and the Pikachu is done in the same art style as Special.
Does anyone have this book or has any idea what this is? Is it a doujinishi drawn by Mato (judging by the copyright and Nintendo's seal at the bottom, probably not likely), or an official book/manga drawn by Mato? A game guide with cover art drawn by her? Looks like something worth owning.