I had a chance to read the book over Christmas, so let me give a quick rundown for those who are interested. The book basically takes the reader past all the episodes and the two movies, in one flowing narrative (so it's not just a list of episodes with some info, but they tried to make a coherent story out of it; mind you, not out of the plot of the series, but out of the book

). Most episodes get between 5 and 30 lines of attention, with a few episodes (like the series finale) getting a bit more paragraphs devoted to them and both films each have their own chapter even. The information usually consists of a little plot summary and some actor/writer/director quote about how fun or hard or whatever it was to work on this particular episode. Some of these or fun to read, some are a bit bland and sometimes they get a bit repetitive (but that's avoided for the most part). All in all it was a fun read, but there was not much terribly shocking in there. But if you like the series and want to immerse yourself a bit in the world of Chris Carter and co then it's fun. It's not a terribly deep analysis by any means and also not particularly critical most of the time, but it's fun. And the book looks wonderful, with many many many beautiful pictures, both from the episodes/films and from behind the scenes. And sometimes there are some fun little hidden things, for example on one page if the light hits under the right angle, a package of Morley sigarettes appears.
There are also some annoying mistakes in the book, ranging from typos, missing pictures (in that there is a description of a picture, but the picture is nowhere to be found, or a different picture is in it's place) and even a missing episode (I noticed that at least the episode "Emily" was missing from the narrative, a bit strange, since the companion episode of this two-parter "Christmas Carol" was in there; I didn't check whether more episodes were missing). And I've not been able to discover any of the "explanations of unsolved plots" that were promised in the back cover text.
There come some little extras with the book: three posters (of both films and Mulder's famous "I want to believe" poster), some reproductions of little plot cards Carter used for "The Host" (these cards just have some plot line written on time, like (paraphrased) "the toilet gargles and a white humanoid monster rises from it"), and an issue of "The Lone Gunman" newspaper, containing some ridiculous conspiracy stories about the government collection our dna via airport toilets waste disposal and such things. These are fun extras, but nothing to write home about (and still I do write about it for you guys and gals

).
All in all a fun and beautiful book for those who like to read a bit more about the series, but don't expect a in-depth analysis or some ground breaking behind the scenes stories. And it's too bad that the many little mistakes give it all a bit a rushed feeling (it was probably rushed, since some of the material in it is very recent). Otoh, things like missing pictures also add a bit to The X-Files mystique.
O, and for those who were still wondering, it's official (if it wasn't yet): there never was a preset grand plan for the mythology. It was made up as they went.
