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Like Barry, I've not tested this conclusively; however, I'd be very surprised if natural comma usage has much if any negative effect.
In this instance you'd probably be better off with a comma splitting the two (very similar) keywords just for the human factor: "cheap books free books" looks spammy and is likely to put people (and potentially Google, depending on how far you take it) off.
Like Barry, I've not tested this conclusively; however, I'd be very surprised if natural comma usage has much if any negative effect.
In this instance you'd probably be better off with a comma splitting the two (very similar) keywords just for the human factor: "cheap books free books" looks spammy and is likely to put people (and potentially Google, depending on how far you take it) off.