I'm especially annoyed by the people who "have read all of Tolkien's books" but also object to a prequel because it's not an adaptation of his work.
Until now, it has been hard to get a good grasp on the Second Age since it's been covered in bits and pieces across
The Lord of the Rings (appendices),
The Silmarillion ("The Akallabeth" and "Of the Rings of Power", which is obviously where the show title came from), as well as parts of JRR's posthumously-published works edited by his son Christopher:
Unfinished Tales (Part 2 is entirely about the Second Age),
The Lost Road ("The Fall of Numenor" and "The Lost Road"),
Sauron Defeated ("The Notion Club Papers" and "The Drowning of Anadune"), and
The Peoples of Middle-Earth (bits and pieces, mostly just material that was cut from what eventually became the LOTR appendices).
A new book
The Fall of Numenor collects these previously published works into a single book. (I think it contains them all, but I haven't actually purchased it so I can't verify it's 100% comprehensive.) It is published in the same style as
The Children of Hurin,
Beren and Luthien, and
The Fall of Gondolin books telling stories of the First Age, although the cover looks more like
The Nature of Middle-Earth (published last year) which is more similar to
The Peoples of Middle-Earth and is written more like an encyclopedia than a narrative.