> Sign language […] does have something map-like that the author writes about.
Not really, no. Sign languages have basically the same structure as spoken languages. (Exceptions are fairly minor, e.g. the use of space for anaphora in ASL.)
> That the language does not have written form is not helping.
Again, no. There are a number of well-established sign language writing systems, including SignWriting [0] and Stokoe Notation [1].