My second thought is that the stuff from "VII. Internals" that I categorized as reference material should move into section "VI. Reference". I think we should also consider moving appendix F, "Additional Supplied Modules and Extensions," and appendix G, "Additional Supplied Programs" to the reference section.
For "VI. Reference" I propose the following Chapters:
SQL Commands
PL/pgSQL
Cumulative Statistics Views
System Views
System Catalogs
Client Applications
Server Applications
Modules and Extensions
-- Remove Appendix G (Programs) altogether and just note for the two that are listed that they are in contrib as opposed to core.
-- The PostgreSQL qualifier doesn't seem helpful and once you add the additional chapters its unusual presence stands out even more.
-- PL/pgSQL gets its own reference chapter since we wrote it. Stuff like Perl and Python have entire books that the user can consult as reference material for those languages.