I bought this game along with Portal and Portal 2. I read a little bit of it earlier but I really read a lot yesterday (I was on a plane, hence the low playtime,) and even finished it. Portal 2 - The Final Hours covers the development, release and post release of Portal 2 inside of Valve. If you like betas, like the Half-Life 2 beta, this is for you. I also reccomend checking out The Final Hours of Half-Life 2. Look it up online and there is a free version.
What makes this so different, though? For starters, it includes interactive content. The second picture (Destroy Aperture) is my favourite. There is also music that you can listen to, like what inspired the songs, and lots of pictures. There are chapters and polls even.
I really like Valve games and betas, and I thought "WHY NOT?" when I bought this. This is worth $2, even $5 or $10. The writing is great, the interactivity is great, there are interviews, videos, music, polls and pretty much everything that you could hope for in an e-book about Portal 2.
Buy it now.