The PDF document has been confirmed by Valve as legitimate.
There's no question that Valve is far from the norm when it comes to game companies, and it's not just because they launch consistently good games. The structure is different than what you'd expect from any other company, and the employee handbook reveals just how different it is.
The document, which leaked online a few days ago, shows that Valve puts employees in charge of what they're doing. They pick their own projects, move their desks to wherever is most helpful for them, and they can work on whatever they want to within the project. There's very little that resembles a traditional work structure.
Valve stresses that employees shouldn't be pulling 60 hour weeks unless they really feel the need, or they simply want to get some extra things done. Overtime, as the handbook puts it, "indicates a fundamental failure in planning or communication." That's a huge shift from the norm, where game companies often push their employees to cram in as much time as possible as games approach launch day.
Valve, of course, realizes that this isn't the optimal organization structure for all things and includes some weaknesses in the manual including "making predictions longer than a few months out." Dang Valve, way to beat us to the punch.
Check out the handbook for yourself. It's a PDF, by the way, so you'll have to download it.