No, it's clearly not redundant. Compare it with this.
Rugby football, more commonly known as rugby or rugby union, is a sport played between two teams of fifteen players with a non-spherical ball.
The Free Dictionary by Farlex, has a section on quotes from classical literature and other sources using external appearance.
Charles Dickens:
Winkle, being engaged in the city as agent or town correspondent of
his father, exchanged his old costume for the ordinary dress of
Englishmen, and presented all the external appearance of a civilised
Christian ever afterwards.
Wilkie Collins:
Vanstone's youngest daughter ran up to him at headlong speed, with
every external appearance of having suddenly taken leave of her
senses.
Dickens again:
Maylie was every [sic] ready and collected: performing all the duties
which had devolved upon her, steadily, and, to all external
appearances, even cheerfully.
And so on. Go to the link for many other examples. These examples, and many of the others, use external appearance as a flag: there is something beneath the surface -- a difference between the exterior and the interior.
Best Answer
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.
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:
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:
Matter of fact, the shortest way to say that he is a farmer in a relative clause.
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:
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:
That said, it is also a matter of idiom: certain phrasings have become common in certain contexts.