good books on PLO?

[RoundTower]

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Rolf's book is currently #26 in Amazon's most-popular poker books list, and Bob C's is #30. I didn't see any others in the top 100, but I only skimmed. Generally I think Ribbo is right. I've contemplated writing a book before, but I don't think it could be profitable enough to be worth my time and effort, especially given the potential to impact the games I play.

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How many copies do you think Rolf has sold? I tried to find out a while ago, but there doesn't seem to be any way to do it online.