This question has two answers: a simple answer, and a complicated answer.
First, the simple answer the author of this book is clearly fishing for. The author thinks you should avoid using these constructions because, in the example sentence given, changing the subject of the sentence also allows you to change the sentence from the passive voice to the active voice. So, instead of:
Restless is what I would call him.
the author wants you to say:
I would call him restless.
The passive voice is a bugbear for many proscriptive grammarians. Overuse of the passive can be a real problem, but the passive voice, if not abused, can also be a useful and expressive part of the English language. A grammarian might not call these sentences part of the classic "passive voice" (his examples contain mostly predicate adjective phrases, not passive verbs), but to a rhetoritician, which I assume this author is, they have the same effect.
Now for the more complicated (and more interesting) answer. We'll take this in two parts.
First: there is absolutely nothing wrong with using an infinitive as the subject of a sentence.
To know him is to love him.
This is straightforward and idiomatic. On the other hand, using a bare infinitive:
* Know him is to love him
is not idiomatic in American or British English. The author's third example sentence will just sound wrong to most native speakers. Not awkward, but wrong, like using the wrong tense or number. I have no idea why the author would ever have thought a bare infinitive could be the subject of a sentence--or, at least, why this would be an acceptable example of it.
Ajdectives and adverbs can be sentence subjects, but doing this means something. At a minimum, it shifts the emphasis of the sentence.
Was she pretty? No, not pretty. Striking is what I would call her.
This is a perfectly idiomatic English construction; but it has a different feel, and a different emphasis, than:
Was she pretty? No, not pretty. I would call her striking.
In other words: the real answer is that the author wants you to avoid these passive or passive-like constructions because he or she is trying to oversimplify a complex feature of English grammar.