The Final Hours is not a game (though can't you imagine one - set perhaps during the Seven Hour War, trying to escape as the facility collapses and the Combine invade the test chambers?), it's a book. It covers how Portal was created, the complex gestation of the sequel, and the personalities involved at every step.
The book is interesting, revealing a lot of behind the scenes info. You learn what a complicated business creating a computer game is, especially a Valve one, and something of the stresses involved. For even at a company like Valve, creating a game is hard work, and it takes its toll. The book makes what use of its computer nature as it can, embedding videos and interactive content. They do not add that much overall though - the pictures and words are what matter.