Joe Zhang wrote: I think a ghostwriter is like a sub-contractor who stays anonymous doesn't get any of the credit.

Exactly.  If I publish my first novel as Edward Stark (shouldn't be too much of a stretch to figure out how I came up with that), that's a pen name.  If JB asks me to write part of his latest novel, but my name doesn't appear anywhere on the work (pen name or real name) then I'm ghostwriting.

My first guess is Powerpuff Girls, but...

Si Spencer looks like a fake name "Suspenser?" and JB has said that DC has given him free reign to deal with the magic characters of the DC Universe. At the time, it looked like he was talking about bringing them into Blood of the Demon, but Books of Magic would be another place to do it. And Books of Magic: Life During Wartime is a sort-of reboot...

Ron Farrell wrote: Just to help clarify the difference: ghost writers are frequently used by people who wish to write their "autobiography" but are incapable of actually writing a book, such as a hot athlete or actor. They are not credited on the book.

Nowadays a Ghost written book will often be credited like this:

by   John Smith

                        With

                       Barry Jones

Or even : by  John Smith

                                         as told to Barry Jones