Valve recently released this [3.9MB]. Definitely worth the read to gain some insight of how the company is run.

Also, there's a picture of a guy wearing a HL3 shirt...

Source
On Friday, a user on the Flamehaus forums uploaded a very convincing pdf, complete with accompanying photo, of a new book called Valve: Handbook For New Employees.

Joining the pdf is an apparent email from Valve's Greg Coomer, saying it was written to "make it as easy as we could for new people to join the company".