Mary Catelli I would recommend reading them in order if only for the spoilers this necessarily entails for the first series.
Silea Yes. I haven't read them, but i'm trying to read this one, and Tod's story is repeatedly interrupted by unintelligible interruptions from Septimus Heap. If i'd read the first series, i might have SOME idea what was going on, and thus not feel like it was so uneven.
George I seem to have inadvertently skipped the last two of the first series, and several times noticed the effect. This really is best treated as Septimus Heap #8.
Shawnette Hill I don't think it's necessary to read the first series as the author does a good job of explaining who the characters are. I would recommend that you read the first series simply because you find this series interesting and it is a tie in to the world of Septimus Heap and would be a more complete journey through this new one.