There is very comprehensive documentation on the Spacemacs website and that same information is also built in to Spacemacs itself, so you can always access it when you offline.
Spacemacs also has built in documentation via:
If you like to see Spacemacs in action, then you should watch the series of Spacemacs ABC videos on Youtube. These videos walk you thought the common features of Spacemacs in alphabetical order.
Most of the classic documentation about Emacs is also relevant to Spacemacs, after all its Emacs underneath. Many of the packages used in Spacemacs are also documentation on the Emacs Wiki, so its devinately worth browsing (especially if you are going to customise Spacemacs).