I came a bit late to the Septimus Heap party--the series was already up to book five (Syren) by the time I read the first book (Magyk). But it was a pleasure playing catch up, and the good thing about coming to a series late, of course, is that you don't have to wait too long between books. That being said, I felt like I waited plenty for the sixth book, Darke (released this June, mg, 656 pages)--I was looking forward to it lots!
I won't try to summarize what happens to Septimus and co. in Darke--if you haven't read the series, it will make no sense, and if you have, but haven't read Darke, you'd probably rather just read the book yourself! So instead I shall offer General Thoughts.
These books are epic at the small-scale level of personalities and happenstances, but with enough larger-scale world building to make the small stuff matter. A lot Happens in these books. Darke, for example, takes place over the course of just a few (difficult) days, but still one gets the sense that 656 pages was barely enough for Sage. The reader get an immensely detailed account of countless threads of plot. And on top of that, she has a large cast of characters, both major and minor, and all of them, from the Princess Jenna to young, mostly irrelevant Maizie Smalls, from the dragon Spit Fyre (who plays a pivotal role) to four orphaned ratlets (who don't), are given time in the spotlight.
All of this detail was of interest to me too, but I think this lush profusion of characters and their concomitant minutiae might be a bit much for some readers--it doesn't always move the plot at hand forward. I myself am a fast reader, and over the past five books I've come to care about these people, so it was fine with me.*
That being said, Darke does have a strong, nay even exciting, plot, that moves forward inexorably toward the nail-biting end! I liked it lots.
*viz character vs plot-- my own imaginary world building consisted mainly of drawing every inhabitant of a pseudo-Medieval town, and writing their back story. None of them ever did much of a plot-like nature; few of them even get to do anything in their portraits....So I'm very sympathetic to Sage's panoply of personages!