Pokemon Encyclopedia is a book published on 5 April 1996, roughly a month after the release of Red and Green. It was the first Pokemon encyclopedia of its kind, and its creation was supervised by Pokemon producer Tsunekazu Ishihara. The artwork in the book was provided by Game Freak. The release of this book predated the beginning of the production of the anime by half a year. The book is notable for featuring a completely unique Pokedex, and pieces of in-universe info which has since never been reused in any other Pokemon product.
At the end of the book is an interesting interview regarding the development of Red and Green, as well as some really cool photographs of Pokemon's dev team.
In creating them, DidYouKnowGaming commissioned Pokemon translator Nob Ogasawara to translate all Pokedex entries into English, resulting in what you could call a semi-official, partly translation of Pokemon Encyclopedia.
As with all books on this site, Pokemon Encyclopedia is rare, out-of-print, and exclusive to Japan. Curiously, however, the book is printed in Western style, instead of the traditional Japanese style. In other words, the book's text is to be read from left to right then top to bottom, instead of top to bottom then right to left. Perhaps an indicator that a Western release was considered?