I first heard of George Orwell's Animal Farm in high school when a teacher mentioned that it was an allegory of the Soviet State. So I read it two years later with that understanding.
While recently reading another novel on the same subject I was led to wonder: If I had come across Animal Farm at age 12 in a second-hand bookshop in India without any pre-knowledge of its allegorical significance might I not have read it like a regular (if highly imaginative and quite sinister) children's story about talking animals? And might not somebody else read it thus without being aware of its political significance?
That confusion would not occur if that information is explicitly stated in the book itself. The Wikipedia article on Animal Farm doesn't seem to clarify this point, but does mention that Orwell's very explanatory 'Preface' was for some reason not published in most editions.
Orwell originally wrote a preface complaining about British self-censorship and how the British people were suppressing criticism of the USSR, their World War II ally (...) Although the first edition allowed space for the preface, it was not included, and as of June 2009 most editions of the book have not included it.
So did George Orwell or the publisher actually state anywhere in the text that it is an allegory?