I just finished reading Frankowski's Conrad's Quest for Rubber, and I must admit to being pretty disappointed with it. The first thing that hit me about it was that it was regurgitating the last couple of books from a different point of view. Then came the anachronisms (that is, the use of terms that would not be every-day ones to someone from that time period used as if they were normal).
I think the final part was that Josip just wasn't all that interesting as a main character.
What I did like about it was that disease factored into it (anyone familiar with Jared Diamond would be expecting that). I thought the diseases hitting the explorer's lasted an extraordinarily long time (I can't imagine how explorers in our world would have survived being out of commission for anywhere near that long. For reasons of not having food, if nothing else), and found the resolution of that a bit... unlikely, for many different reasons.
So I guess, if I were just finding the series now, I'd stop at five.