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:
SPCmeans press the spacebar.
M-mmeans hold the meta key (Alt or Command key) down and press the
M-xmeans hold the meta key down and press the
Spacemacs in action
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.
There are also short screencasts of Spacemacs that accompany this book.
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).