"Elasticity of marginal utility of consumption ( μ ) with respect to utility." (Green Book quote)?
Mere googling leads me to find a bunch of references about elasticity of marginal utility. 

Here Professor Acemoglu defines the elasticity of marginal utility as the inverse of the intertemporal elasticity of substitution (pagg. 17-18).

Here you can find a question analogous to yours together with an answer explained in an easier way than Acemoglu's lecture. 

If you are interested in something more empirical, have a look here: it is a paper by David Evans about the empirical estimation of elasticity of marginal utility in OECD countries.

Hope this is enough. If you have other questions, please ask