The interesting thing for me was that there's books from a larger collection of works, like those from Tolkien, C.S. Lewis, GRRM and Tad Williams. Even keeping in mind that this list is simply one of a popularity thing from the folks at Goodreads, I'm surprised they're not collating the collective popularity of the entire set vis a vis:
Tolkein's LOTR is really a four book series, with The Hobbit, Fellowship of the Ring, Two Towers and Return of the King
C.S. Lewis
The Chronicles of Narnia is really a 7 book series, with Lion, The Witch and the Wardrobe, Prince Caspian, VOyage of the Dawn Treader, The Silver Chair, The Horse and His Boy, The Magicians Nephew, and The Last Battle
GRRM - A Song of Fire and Ice is (allegedly) 7 books: Game of Thrones, A Clash of Kings, A Storm of Swords, A Feast for Crows, A Dance with Dragons, and many fans awaiting the release of The Winds of Winter and A Dream of Spring.*
Terry Brooks - Shannara Series (at least the initial installment) was really a trilogy (a la Tolkien) with The Sword of Shannara, The Elfstones of Shannara, and The Wishsong of Shannara).
Tad Williams - The Memory, Sorrow, and Thorn was also a trilogy of books: The Dragonbone Chair, Stone of Farewell, and To Green Angel Tower
*****
To simply pull one book from what is clearly meant as part of a larger collection and call it "The One" I think does each author an injustice. And I'm just picking what are likely the most popular
series of books that are listed here. These ones alone would make up 22% of the top 100. Does that mean stripping other authors out? Probably, but I'm ok with it because these writers really do grab that much of the market. I think that's also a fairer metric, because for me once I read one book in a series, there's a compulsion to find out the rest of what happens!
So, that means if you hook me in your first book of a series, I'm smitten for the rest.
And finally, if you're going to include a series as part of this popularity metric, am really surprised like others have said that the omission of the Harry Potter collection clearly displays some subjective criteria in even creating this "list" in the first place.
Be that as it may, if you'll excuse me, I need to keep reading. If I can finish my goal of the 100 canonical books, 100 Sci Fi books, and 100 Fantasy books, I'll likely start making a claim that I am finally a well-read individual
*I'm finally done with what's been published though, so will permit myself to view the HBO series now...