"That Is All" is an excellent book, but really hard to write about. It's a funny book, but many of the most memorable parts come from a very dark and disturbing Cthulhu-inflected mythos. It doesn't really have a plot, except that it does. It includes long lists that tend to be amusing as snippets, and hilarious in aggregate.
The book follows the same general form as John Hodgman's two earlier compendiums of knowledge, "Areas of My Expertise" and "More Information than You Require." They tend to read a bit like almanacs, with a collection of short pieces offering advice, or relaying anecdotes, or categorizing certain items, or whatever. Where the first two books summarized all existing world knowledge, That Is All completes all world knowledge, by explaining everything that happens between now and the end of the world, which will happen on December 21st, 2012. (Arguably, it's actually over on the 20th, but technically Ragnorak itself happens on the 21st after the human race and most of the planet has been extinguished.) A lot of the book deals with the end times. Every single page has a section from "TODAY IN RAGNAROK", an Almanac-ish prediction of what will happen on that day. These are often short, just a sentence of two (Jonathan Franzen delivering a mysterious manilla envelope to a Hollywood celebrity, for example), but they often accumulate over multiple days and weeks into fairly epic stories; and, all the stories together create the horrifying, tragic picture of the world rushing towards annihilation.
The body of the book covers much more ground, including hilarious bits on foreign etiquette, ocean cruises, types of wine, and so on. However, it also includes an astonishing number of pages listed to enumerating the names of the Ancient and Unspeakable Ones. (This part is very reminiscent of the Hobo-focused lists of Areas of My Expertise, but way more disturbing, which remaining mostly funny.)
There's also a strong autobiographical slant to the book. As usual, much of what Hodgman writes about himself is fiction, but particularly at the beginning and the end, he reflects a great deal about the success that he's found in life (through the earlier books, his role on the Apple commercials, and stints as a character actor on TV shows), what parts of it have changed him, in what ways it's been transient or fleeting, etc.
This is a strange book, in keeping with the previous two, but definitely worth reading. You have a little under a year until the world ends, so if you'd like to give it a shot, now's the time!