To say the least, this is not a book for the mathematically faint of heart.
m6nnich demonstrates how to use a kind of "macro grammar" to define tree languages in mSO for the non-context-free phenomena that one can find in natural languages by replacing the normal substitution process that rewriting systems assume in their notion of derivation by a higher-order variant in which operations such as composition can be explicitly represented as objects in the macro language.