Since adding a prequel serial to the Wolf Rampant series, I’ve had several readers ask, “Which do I read first? The serial or Shiftless?”
I remember having the same problem with Narnia series. There, and here, I ended up deciding it made the most sense to read the books not chronologically by the standards of the fantasy world, but chronologically by the standards of the author. In other words, I recommend reading Shiftless first.
On the other hand, you’ll definitely see Terra’s world through different eyes once you’ve spent a while hanging out within Wolfie’s mind. In fact, I had one reader email that she read the trilogy then the serial then wanted to go back and read the trilogy again because the whole story looked a bit different after our favorite bloodling alpha had his say. So if you want to slide The Complete Bloodling Serial in between Shiftless and Pack Princess, I don’t think that’s a wrong decision.
But, yes, probably read Shiftless first…which is easy since it’s free.