This two-volume work on syntax views grammar as a non-arbitrary language-processing device, to be understood in terms of the various substantive parameters relevant to communicative function, cognitive processing, socio-culture and neuro-biology. Distortions in this complex yet highly iconic code are due to conflicting functional requirements, most commonly introduced into the code through the course of diachronic change. Cross-linguistic variation within each functional domain is highly constrained and yields a coherent typology of the most natural ways the same communicative functions can be performed.
This book was required in one of my linguistics classes. It is very thorough and includes examples from many languages. I learned a lot from it. There were times that I wished it gave a few more examples so that I could be certain I understood. I used another book to help in those cases.