This is your Apocalypse, would you like to Super-Size that? Cthulhu Apocalypse: The Apocalypse Machine is a sourcebook full of options for Trail of Cthulhu Keepers to customize an end-of-the-world-and-beyond campaign, dealing with character creation options and maintenance rules in end times, new Abilities (including psychic abilities and other weird powers, which may be useful in certain other campaigns as well), and a shelfload of plug-and-play elements for different types of apocalypses.
The Stability and Sanity rules are particularly important because they allow Investigators to go on ticking even when their Sources of Stability are dying off and their Pillars of Sanity are crumbling - these issues create complications without rendering an Investigator utterly unplayable, in The Apocalypse Machine.
Graham Walmsley writes with his usual dark confidence and style about every aspect of a 1930s apocalypse you can expect to read about, including the effects of worldwide breakdown going forward up to 100 years later. There are also writeups about how various Mythos entities and monsters can play into apocalyptic settings, including the worthwhile inclusion of H.G. Wells' Martians and Red Weed.
An excellent toolbox for end-of-the-world settings, from The Road to The Death of Grass. I may even borrow the psychic powers rules for other ToC games...