Is it legal if I wrote my own sequel to this book I read?

Technically, no, it's not legal, but no one will detect it or have reason to pursue you if you do not publish it anywhere (which includes fanfiction, as fanfics are actually illegal, it's just that they're normally let slide because they're so small-scale. There have been cases though of both sides suing each other).

EDIT: Fanfictions are not legal because they are not commentary or review of copyrighted worlds/characters/names/etc, they are the actual use and recreation of them. But most authors and houses ignore them because they're not for-profit and are made by fans. However, many houses and publishers are now explicitly stating that they do not approve of fanfictions and they are illegal, because recently there have been a few cases of the FANFIC authors actually suing the real authors when the real authors release another book/movie/episode that has work that resembles something from the fanfiction. So authors are protecting themselves by now enforcing their rights and avoiding the whole mess. Some others kind of wink and pretend they don't know about it. Try and research what side this writer falls on. But, you know, this isn't really fanfiction- you're talking about a full-scale book. I'd only risk it if you're absolutely going to keep it to yourself and/or just a few close people. But why waste that much work on something you can't freely release or be rewarded for? If you've got that much imagination and determination, write your own original work! Good luck.

It has been done before, presumably legally. There was a book a couple years back, I don't remember the title, which was the story Gone With The Wind told from the slaves' point of view. There is Wicked, which is another point of view of the Wizard of Oz. You should have no trouble doing this with a book whose copyright has expired. If the author is still alive, you may need permission from them if you want to publish it or otherwise make money from it, but online available for free? Shouldn't be a problem. Make sure you do give some sort of credit to the author of the original story. Edit: What you are talking is satire, and yeah, it sure is legal! A couple years ago I read a book by Chris Elliot that satirized, among other things, The Da Vinci Code.

Sure write your own sequel! Just don't publish it, put it on the internet, anything like that. If something like that occurs make sure you contact the author telling her you are in no way going to give this to the public. Have fun!

I do this ALL the time to Harry Potterr!!!

it's only illegal if you publish it, because of copyright. If by some miracle you got THROUGH copyright, then it'd be OK. Otherwise, if it's just for you a couple friends and family, then go right ahead!

Yes

If you aren't publishing it and just writing it for your own entertainment, yes it is legal. That's fan-fiction, and is quite popular.

it's called Fanfiction. as long as you don't make money on it, and clearly say that someone else wrote the first one, you don't own the ( setting, characters, whatever), you can even post it online for other fans to enjoy.

It's perfectly legal, and if you want other peoples advice and admiration you can post it on http://www.fanfiction.net/

do you want to go to jail? JK, there's nothing wrong with it, go ahead write what you want.

Who cares? You aren't selling it, screw anyone who thinks you shouldn't do it.