Jose, icao Level 6 English, is for expert speakers, not necessarily Native but darn close.
Judging by your current grasp of English, although good, it is quite away from Expert.
I understand you do not wish the pain of having to retake your Level 4 exam, but you really should not be thinking a book can get you Level 6.
The rules are there to make aviation safer, I have flown with Captains who's language skills were barely level 3 and in some parts of Africa ATC is worse.
I would not worry too much if I were you, you seem to have a good grasp and will pass level 4 with no problems.