Dear Lisa Lutz, What do You Call that Person You're Spying on?

We're working on our Lisa Lutz event, which we're hosting on March 25th, 2010, at 7 PM. I'm in the middle of her first, Curse of the Spellmans, while Anne is ahead of me, having already finished her newest, The Spellman's Strike Again.

Here's what Anne has to say:

"The fourth and final installment of the Spellman Quartet finds the Spellmans spying on each other more than they are for clients. Mom is keeping tabs on Isabel’s dates in the hopes that she’ll get married. The kids want to know why parts of the house are disappearing. Could someone really be stealing their doorknobs and light fixtures? Lutz has written a quirky and sometimes bizarre tale that’s good for a laugh."

Here's our problem. What's the word for the person that a detective is spying on? I hire a detective to see if my wife or husband is having an affair, or my business partner is cheating me? What is the word you use to describe them?

I've done a bunch of searches but can't find anything appropriate. "Mark" just doesn't seem correct. That's someone you con. What in Miss Marple's name is the right word for this?

I also usually tell booksellers to hold their posts until the new book is out, but in this case, you might, like me, want to start with book number one, The Spellman Files.