A battle of opening books on otherwise equal chess engines might be a way for humans to remain relevant in chess competition at the highest level. The opening is where long term strategic decisions get made, supposedly a human strength over computers.
We need some mechanism to inject randomness, especially for a head to head match. The opening book itself might give several equal moves. Multi-threading in the engine might cause randomness. Or the engine could be programmed to randomly choose between moves with the same evaluation.
We might also need a mechanism to handicap weaker engines, in case an opening book author prefers a weaker engine. Easiest is probably to compute the opening book quality as a delta performance improvement over the engine's performance rating with its default book. Otherwise, time advantage.