What is the largest work that has been published using LaTeX?

The perhaps biggest document set with LaTeX, that I am aware of is Christoph Schiller's Mountain Motion collection. It consist of 6 volumes, with 566 + 376 + 430 + 286 + 442 + 456 = 2556 pages in total.

Another extensive work, entirely typeset in TeX is Donald Knuth's The Art of Computer Programming. Without the supplemental material the volumes 1-4A amount to 650+20 + 762+14 + 780+14 + 883+16 = 3139 pages in total.

The longest document I know of is the Erco catalogue from (now with a broken link) Showcase of beautiful typography done in TeX & friends answer of Stephan Lehmke. It was a mindblowingly dense document but he uses his own system Docscape (again based on TeX if I'm not mistaken). If not or if Stephan wants to write his own answer, let me know and I can delete this

2016 version of the document is here to get an idea how it was. More examples are in his website.

I create the SAS/STAT User's Guide complete with output tables, graphics, mathematics, bibliographies and two indexes.

Currently it comes in at 10,425 pages.