Tolkien Estate blocks new Tolkien book

[Lalaith]

The copyright in these private, unpublished letters belongs to the Tolkien Estate. As the guardian of these rights and of the privacy of the Tolkien family, both of which it takes great care to protect, the Estate quite properly declined permission for the letters to be reproduced in this way.

I'm still a bit baffled by this. Firstly, because most 20th century literary figures eminent enough to have biographies written about them, have letters that are published as part of biographies, and it isn't generally seen as a problem by the estates of these writers.
Secondly, from whom did the authors obtain the letters in question, if not the family?
Thirdly, why could the letters not be paraphrased? If I paraphrased the plot of LotR I would not be in breach of copyright, although if I copied it word for word I clearly would.