Continuing A Series After The Author's Death

The Hitchhikers Guide To The Galaxy, The Wheel of Time and The Millennium Series, all have something in common (besides being on my bookshelves); after the death of their authors, someone else has continued the series. Why does an author feel compelled to do that, finish someone else's series? Is it love? Why does a publisher go ahead with this? Is it money? Is it also love?

Though all these series are in my home, I haven't read them all. The Wheel of Time is my Hubby's. I'm not sure if I'll read it one day or not. I want to, but the books are huge! I have read The Girl With The Dragon Tattoo and recently The Girl Who Played With Fire (finally), but not The Girl Who Kicked The Hornet's Nest. I don't know why, I suppose I just haven't gotten around to it. I have read the five Hitchhiker books and loved them. Was I a fan of the ending? Not necessarily, but it felt like an ending; it was complete. When I heard about Eoin Colfer's book And Another Thing, as a continuation, I was excited and hesitant. I managed to pick it up (a long time ago - my son was an infant) at a charity book sale. It still sits unread on my shelf.

I've been thinking about this lately. I've also been making an effort to read the books that have been on my shelf for a while, like And Another Thing.  I'm so hesitant to read it, but I want to and I want to love it.  I'm concerned that my expectations are too high. I know Hitchhikers isn't for everyone, but I loved it. What if I hate this "sixth" book?  Would it ruin the series for me? Would my ire be worse than normal? Am I over thinking this?

What do you think about authors who continue a series?  I know I mentioned The Wheel of Time, but that was a different case, right? From what I understand, the series was clearly not finished and Brandon Sanderson worked from Robert Jordan's notes. Someone correct me if I'm wrong.  That is not the case with Millennium, as the author is creating a completely new work, not using the partially finished manuscript Stieg Larrson's girlfriend controls (not that I expect anyone to ever have a chance to finish that novel).  I don't know if Douglas Adams left notes for another book.  Did Eoin Colfer write a book to give himself and fans a better ending to the series? I thought the end of Adams' fifth book was pretty final.

I should just read them, right? No? Do I want to read David Lagercrantz's The Girl in the Spider's Web? I don't know. Yes and no.  If I ever decide to read The Wheel of Time, I will read the books written by Sandersen. That case is different, right? I would go into the series knowing it would be finished by someone else. I finished Hitchhikers years before Colfer wrote his book.  I started reading Millennium as a trilogy. I love reading, and I want to give authors a chance. My fear is that they will change how I feel about Larsson and Adams' work. Though at the rate I'm going, it'll be years before I get around to reading these books.