By the mid 80s, the TSR staff had run out of new ideas, so they started packaging up collections of their older works into "supermodules". S1-4 Realms of Horror supposedly collates the classics S1 Tomb of Horrors, S2 White Plume Mountain, S3 Expedition to the Barrier Peaks, and S4 Lost Caverns of Tsojcanth ... but it is a bit of a butcher job. S1 is presented with all sorts of small little changes, like the wording of the message in the path, or getting rid of the fixed sphere of annhiliation and turning it into a zombie producer, and frankly I find these changes unwelcome. S2 is the closest to the original, with a helpful chart for the geyser room that the original lacks. About half of S3 is present, but the all important ending fight to stop the worker robots from throwing more monsters into the wild is not, as well as all the interior artwork, so I'd rate this portion of the book as unplayable. S4 has the second booklet sensibly chopped off, saying you can just get all the monsters & spells from MMII and UA, and I'm okay with that. It rearranged other things, has some editing fails labeling the sections wrong, but also provides some handouts that in the original were just expected to be read aloud. However, like the other modules in the book, most of the artwork from the original is missing. All in all, you are better off getting the originals or getting the new S1-4 compilation that just came out in 2013.