The official Squirrel language documentation can be found at squirrel-lang.org (the imp uses a slightly modified version of Squirrel 3.0.4): Squirrel Reference Manual
All Squirrel programs on the imp will need to use at least some of the Electric Imp API, the objects and classes that encapsulate the imp hardware and its internet connection. Some may also need parts of the Squirrel standard libraries, also documented at squirrel-lang.org: Squirrel Standard Libraries Manual
There are a few differences between the standard Squirrel libraries and imp Squirrel's libraries. The biggest one is that the Squirrel math library is in its own namespace, math. So where the official Squirrel documentation talks about, say, acos(), on an imp that would be math.acos(). Other differences are more minor and relate to operations that can't work on an imp: all the file handling calls, for instance, have been removed, as there's no filesystem on an imp.
Present on the imp
Absent from the imp
getroottable assert print error array type callee dummy