Thirty years ago (give or take a year) my best friend in grade school brought a copy of this book for me to look at. Twenty years ago (give or take a few years) I paid my friend a visit at his university, and spent a morning leafing through his own personal copy of the book, looking again at the entries on gremlins and vampires, Frankenstein and his monster, werewolves and witches, King Arthur and Camelot, and those islands visited by the likes of Lemuel Gulliver and Sindbad the Sailor. When I went back home, I eventually special ordered my own copy from Super Crown.
At first blush, it is not dissimilar to other books wherein you can read up on the gods and beasts of myth and folklore, mysterious lands in the remote corners of the map, and other such things. But it's Page's prose that adds a touch more to the subject matter, treating these topics as if they are, in fact, real. Positing that Treasure Island isn't that far from the Carolinas. Hoping that the Brobdignagians haven't discovered gunpowder. Hinting at what sort of leather Satan's briefcase is fashioned from. "Encyclopedia" is in the title, after all.
I have two minor quibbles with the Encyclopedia:
1. The author limited entries to myth, folklore, and literary creations. Which means there is nothing from film. There is an entry on gremlins, yes, but these were the creation of Roald Dahl; the Encyclopedia adds them to the family tree of leprechauns, elves, gnomes, goblins, and other mischievous "little people". On the other hand, the illustration accompanying the entry on a certain vampire count looks a great deal like the film actor best known for portraying him. But if you want to read about Skull Island, you'll have to check out The Dictionary of Imaginary Places.
2. The Bibliography is extensive and informative, but there is one entry that continues to bug me because I cannot place its origin. In the section on "Things of Magic, Science, and Invention", there is the entry about "Anonyma", where a scientist brings the disembodied head of an executed criminal back to life. Predictably, it ends badly for all concerned. It seems like a variation on the story of Frankenstein, but his entry is a little further along in the book. My guess is that it's something the author himself came up with, since some of the sources listed in the Bibliography are books by he and his illustrator, like The Voyage of the Poppykettle.
I've had my copy of The Encyclopedia of Things that Never Were for over twenty years now, and still look into it every so often, alongside The Flight of Dragons and the aforementioned Dictionary of Imaginary Places.
This is an absolute must-have for any fantasist's library.