I think the Dresden series just keeps improving, which is awesome.
The world building/magic systems keep getting better, more solid, more complex. The characterizations are much more thorough and deeper, not quite as one note and corny.
Harry is still Harry, a little irritating sometimes, but he's still learning without becoming some ZOMG! Powerz!!! character which might happen by the fourth book in any other fantasy series. It's a solid Dresden book, but better.
Bonus cookies include the return of a lot of veteran characters that I really like and the addition of some new ones (most notably other Knights of the Cross and the Archive)