I would not recommend a single book for everyone to read, because people have different tastes and interests - and even if you recommend a single book based on people's interests - if they are really interested in that and the book is important to that interest it's likely they've already read it. So probably a recommendation on any field should be made based on calculations both of how essential it is, but also how it's reading may change assumptions about the field if the person has not read it. It seems somewhat arrogant. And in the end it's annoying to get recommended a book one considers essential to the field, as it implies one does not know what one knows. All that said - for the field of: Fantasy - Gormenghast America - Democracy in America Programming - Structure and Interpretation of Computer Programs
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