Having read the "unofficial sequel" Flatterland before, I was a bit disappointed by the very low amounts of science and mathematics in this book. But, on the other hand, it verges more towards philosophy and sociology.
If you always keep in mind when this book was written, it serves as a nice satire of the thinking in that time (and even present thinking in some cases).
Don't read this book for the plot, because it is very thin, but this is not really a big issue in this case.
The book loses a star because of the frankly horrible "Preface to the second edition" where the author (sorry, the editor who speaks for the anonymous square who wrote the book) tries to answer to some of the criticisms he received - by denouncing some of the very ideas of the book and apparently failing to see the newly opened possibilities for satire.