Jessica > Status Update

I technically have the fifth and final book in The Time Quintet...but it's the one that features the daughter of Meg (Murry) O'Keefe and Calvin O'Keefe, whom I know nothing about. There are earlier books that would allow me to learn more about her...but they're not part of the series. So, do I go ahead and read the fifth book now? Or do I order the other books and read them first?!