Large tracts deal with the written word, especially a variety of linguistic reviews and papers, as well as a great many dictionaries, phrase books, books of quotations, etc.
Which means his sentences aren't the kind you would expect to see in your average phrase book -- they're flowery, don't follow grammatical norms and are open to interpretation.
After that, it was about going through the primary source materials and pulling out sentences and phrases and organizing them in a phrase book for different situations.
