Azrael360: Also, the manuals (for the games who have them) can be directly downloaded or read from the store page of each game. The links can be found in the bottom-right of each page (as "View the manual"). You don't need to have bought the game or have logged into Steam to download them, they're there as a public download for everyone, unlike the manuals for games on GOG...
As an example, see the store page for Skyrim:
http://store.steampowered.com/app/72850/
I confirm this feature and it's great to download the manual (most of the time in your language, if it is available), even if you don't own the game.
Indeed, you can grab a french manual for a game you don't own in your Steam library whereas, sometimes, you don't have access to a french manual for your GOG game because it is english only !