I think #16 (the end of vol 3) is a pretty big inflexion point for the series. Paraphrasing, but Ronnie asked elsewhere if there was a point where the series suddenly took off. I think #16 is it, personally. A lot of the series to date coalesces at that point, setting up the remainder of the book. However, again, it doesn't go where you would necessarily expect it to.
And, I'm still of the opinion that if the book wasn't to your taste after vol 1, it's not going to suddenly become so after any of the subsequent volumes.