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Then in the left-hand column, click on "Look & Feel". Highlight the book that you want to convert, then click on "Convert" up at the top.
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When you convert a book using Calibre, you can make all kinds of changes to that book - most of which are totally over my head! - but among those are changing the font. (And conveniently it's also the only format that the FONT_RAMP "hack" for changing font sizes works with, so lucky for me!) You'll be converting your book to the AZW3 format - it's been a while since I started doing this so I can't remember, but if I'm remembering correctly it's the only format that will allow the change.
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You can only make this type of change to a book that has no DRM - and of course DRM-removal directions are not allowed here. Here's how to add a different font choice to your books using Calibre.
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