Section 9.6 describes haemoglobin as an allosteric macromolecule i.e. that the binding of a molecule (e.g. oxygen) on one part of the macromolecule can affect the binding of a spatially distant binding site. Allostery is important for haemoglobin as it must be able to bind oxygen strongly enough at the lungs and release oxygen quickly enough at the tissues. If haemoglobin displayed simple binding properties this would not be possible.
It mentions that CO2 has an effect on the binding of oxygen to haemoglobin. I am told that it is the presence of CO2 in the blood and not that lack of O2 that stimulates breathing. This is a reason often given for people accidentally drowning while holding their breath as they often expel air (including CO2) before diving to increase the dive time.