"Neutral book" bias?

[Kappatoo]

I guess it wouldn't be so easy today because today's top engines do not have such obvious strengths and weaknesses relative to each other (I won't speculate on possible reasons for that here ).

So the potential is there, whether it was actually realized or not is really hard to judge.

Realized by whom?

[Uly]

If you tune the book to choose lines that Rybka likes, how is that NOT tuning the book to rybka?

That Rybka thinks a position is equal, and so, it's picked up by book makers, doesn't mean that Rybka understands and plays better from that position, so Rybka isn't getting an extra advantage from them.

I still think there's bias introduced by this, in where engines that understand and play better such positions are getting an advantage, but I don't think it's possible to get rid of some bias, regardless of the method used to pick positions. As long as the book is varied enough in the themes of the positions, the effect of the bias should be insignificant, though.