Books like "Marooned" and "Echoes" were written before Kes was dumped. But they were published later. As for the events in the books, "Echoes" takes place somewhere between "Futures End" and "Warlord" while "Marooned" takes place after "Before And After" and "Real Life".
As for further information, visit the Kes Website at http://lynx677.byethost12.com/
and continue to the links "Book Reviews" and "Voyager Timeline".
I think that the season 1-3 books are great. Most of the stories are actually better than many episodes. As for Kes, she plays an important role in most of the books.