The guideline is that some adjectives and adverbs are non-gradable: they cannot be less or more so, but just so or not so.
Perfect is one example-- I am either
perfect or
imperfect; supposedly, I cannot be
a little bit perfect or
very perfect.
Your grammar book should contain a representative list of descriptive modifiers that are generally considered non-gradable: words such as
perfect, unique, impossible, dead-- and also adjectives like
Japanese, mechanical, metallic, etc. For others, it is up to you to imagine whether the quality can be graded as 'more' or 'less'.
Having said that, the non-gradability of many words is debatable-- few native speakers pay close attention to this guideline; in addition, there are some intensifiers which modify even non-gradables:
The entree was absolutely perfect! This riddle is utterly impossible!