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That is a free editor/browser from W3C, the web standards body.
Among other things, it has a "book" operation that lets you
combine multiple files into one large one with internal links
adjusted and a TOC based on HTML headers auto-generated.
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EPUB Webinar: Join STC Vice President Nicky Bleiel as she discusses tips for creating EPUB, the file format used for e-readers, tablets, smartphones, and more.