This is a history of human beings' invented worlds, not specifically from fiction but rather (mostly) worlds which human beings invented to explain the unknown parts of the earth, which exploration and science hadn't yet revealed.
The chapters cover worlds such as Atlantis, Shangri La, and numerous other fabled lands. Each chapter is further supported by excerpts from many of the referenced texts.
Eco isn't always obvious in his purpose - though if you have read Foucault's Pendulum, for example, you already know how he feels - at least in the first chapters but his intent is, rightly, to point out the silliness to which human imagination gets when it is not provided with answers that it accepts as reasonable.
Eco is relatively objective for such an easily mockable subject, but he occasionally injects humour and insights into the human condition, which make the book even more compelling.
The one problem with the book is probably the fault of the printer and not Eco himself: the translation contains a couple of errors - don't know whether they are the translator's or the printer's - and the addendum to each chapter isn't always well organized: The texts are usually listed as they are mentioned but not always. Some references in the text are not clearly found in the addenda and a couple times I don't think I ever found them. But, again, I think this was a printing error for the hardcover English edition.