If I recall right, Tokyopop announced that they licensed the Slayers novels. They translated the first six novels and because of the low sales decided to stop there and tried to pass those six first novels as all there is.
This caused a huge shitstorm amongst the most decided fans and spawned a petition for Tokyopop to continue translating. After a while Tokyopop conceded and translated two more novels(7 and) which effectively finish what we know as Slayers Next.
The new upcoming anime, while having anime original plot, will draw at least some plot elements from the novels 9-15.
Fuck thats a lot of tl;dr