Robert Bringhurst: "The Elements of Typographical Style" ISBN 0-88179-205-5

Victoria Squire: "Getting it Right with Type" ISBN-13: 978-1-85669-474-2

Ellen Lupton: "thinking with type" ISBN 1-56898-448-0

Derek Birdsall: "notes on book design" ISBN 0-300-10347-6

I have enjoyed reading all four and I recommend you read at least two of them because there is not one and only one true and proper way to do this. Nor is it trivial so before changing anything you should make sure you know what you are doing! The ease with which the user can meddle with typography is one of the reasons word-processors such as MS Word and OpenOffice Write should be banned :-)

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