To me, it is a mistake, and it should be 'enable'. Is that what you mean by 'it is pointed out'?
To me, the object is '...vocabulary and ... rules'.
I suggest you get a newer book though. The style of that language is rather archaic, as is the sexist 'he/his'. It reads as if it were written 50-100 years ago.
There is another error as well. 'another English speakers'. It should be 'other English speakers' or 'another English speaker'.