Absolute returns describes an investment philosophy and is regarded as the opposite of the relative return approach from traditional asset management.
Asymmetric returns is thought of as the implementation of an absolute return investment approach. The key "insight" is that large losses are not good for one's financial and mental health, as they kill the rate at which capital compounds. A loss of 50% requires a return of 100% just to breakeven.
The relevance for the investment management profession is that most investors do not need external help to compound capital negatively.
The two books are essentially the author's manifesto for active risk management.