Sometimes I find sentences that have the following pattern:
[clause], which is what [noun] is [optional phrase].
For example:
Another one:
Are the two verbs "to be" in the clauses after the comma not redundant? If not, how to make sense which "is" goes to which subject?
No, there is no syntactical redundancy. There are two relative clauses, one introduced by which, the other by what, and each must have its own finite verb.
... the Holy Grail, which is what the book is about.
... the concept of "enablers", which is what the folks around Birnam really are.
You could see these as fusions of two separate hypothetical clauses each:
... the Holy Grail, which is this: that which the book is about.
... the concept of "enablers", which is this: that which the folks around Birnam really are.
Even so, you say, the which is this part could be left out without loss of meaning. That is only apparently true; the construction is used because we want to put focus on "the fact that x is y". This is how it is used in your examples too. The meaning of a sentence is not just the corresponding elements of reality that it refers to: it is also the way these things are presented. Consider the following sentences:
Rome is the capital of Italy.
It is true that Rome is the capital of Italy.
These sentences describe exactly the same thing. The difference lies in the way they present it: 1 is matter of fact, neutral; 2 could add several different shades of meaning, depending on context. "But it is not its richest city": concession. "So you have passed the test": a formal test. Etc.
Similarly with which is what...is:
He talks like a farmer, which he is.
Matter of fact, the shortest way to say that he is a farmer in a relative clause.
He talks like a farmer, which is what he is.
The fact that he is a farmer gets more focus. This may be done to emphasize that this is an important point, or because it was unexpected, as an expression of resignation ("which is, after all, what he is"), etc.
Another factor that may influence this choice of construction is that it is sometimes the easiest way to ensure a certain word order:
It was a massacre. That's what the book is about.
We want that at the beginning of the sentence, because the link of that to the previous sentence must be strong and clear. Notice how much weaker the following sentence looks:
It was a massacre. The book is about that.
That said, it is also a matter of idiom: certain phrasings have become common in certain contexts.
There is nothing wrong with such constructions. To boil it down to basics, in a sentence like
This is who I am.
the object of the first is is "who I am" — a noun clause. There is nothing confusing about this. The confusion comes from long, rambling sentences with lots of clauses that are ambiguously related to each other.
A noun clause can also be the subject of the sentence:
What I am is a writer. What I am not is a carpenter.
