I truly appreciate the mail that I get asking for further adventures for many of my characters.  I'm honored that you like them well enough that you want to spend more time with them.

However...

As much as I enjoy all (well, most of) my characters, once I finish a story, I want to move on to other people in other situations.  Once a book has been published and out for a while, with readers deciding for themselves what happens after the end of the story, it hardly seems fair for the author to come back and say, Nope, you're all wrong--THIS is what happened next. 

Besides that, one of the reasons I became a writer was because I enjoyed mentally writing sequels to books I didn't want to end.  (Or I made up new endings for stories I felt ended the wrong way.) 

It's an interesting exercise that you might enjoy.

In User Unfriendly, a group of kids are playing a Rasmussem Corporation game that goes wrong.  In Heir Apparent, one of those kids--a secondary character in User Unfriendly--plays a different Rasmussem game.  Guess what.  Things go wrong again.

In Deadly Pink, a girl is enlisted by Rasmussem Corp. to talk her older sister out of the game where she has intentionally stranded herself.

The books can be read in any order.

The major character in the Changeling Prince is a secondary character in The Conjurer Princess.  The major character in The Conjurer Princess does not appear in The Changeling Prince.

Although The Changeling Prince takes place before the Conjurer Princess, the books can be read in either order.

All right, all right: I suppose they're sequels, but the books featuring Twitch the Squirrel can be read in any order. (And they are all illustrated by the incredibly talented Steve Bjorkman.)