This is the book I wanted.
In this sentence, is 'this' the formal subject and 'book' the real Subject?
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"this" is the subject, "the book I wanted" is the subject complement. "book" is merely a noun within the phrase that constitutes the complement.
If you invert the sentence to
The book I wanted is this one.
then "The book I wanted" becomes the subject.
They both have very similar meanings, but the emphasis is different.