A horizontal ebook is not finished merely because its paragraphs look horizontal. Its text should flow from top to bottom, its pages should progress from left to right, and its navigation, ruby, and images should belong to that same structure. In a worked four-chapter EPUB, the package declared horizontal-tb and the spine declared ltr. Those two values provide a useful anchor for the rest of the build.
Prepare a book that exposes direction errors
Use four chapters—Beginning, Map, Gallery, and Afterword—with a map in chapter two and a photograph plus ruby in chapter three. Add title, author, publisher, and a cover. If the source began as vertical prose, review full-width Latin characters and special vertical combinations before reusing it.
Horizontal Japanese does not require every number or Latin term to be full-width. Preserve readable URLs, years, and model names. Ruby still needs a precise base-text range and reading.
Set direction before selecting chapters
Choose horizontal writing and Japanese language in the book settings, then select the four chapters in reading order. A conventional sequence is cover, title page, navigation, chapters one through four, and colophon.
The sample navigation retained all four headings and the pages advanced left to right. If a horizontal book advances in the opposite direction, return to the book’s writing-direction setting rather than rewriting paragraphs.
Inspect ruby and figures after conversion
The readings for Tokyo and hydrangea became ruby elements in the generated chapter. The map and photograph became packaged images with their alternative text. This matters because an editor preview cannot prove that the exported EPUB retained the same structures.
If an image is missing, changing writing direction will not restore it. Correct the source-relative image path. If a reading is wrong, correct its base-text range and reading in the manuscript, then rebuild.
Do not judge by one CSS string
The horizontal sample’s shared stylesheet also contained a reusable definition for a vertical class. The chapter body did not apply that class, while the package still declared horizontal-tb and ltr. Finding the string vertical-rl somewhere in a common stylesheet does not by itself make the book vertical.
Read the package direction, body class, and spine progression together. They should describe one horizontal book.
Recover at the source of each failure
Return to writing-direction settings for reversed progression, chapter selection for incorrect navigation order, image references for missing figures, and the manuscript for faulty ruby. Do not patch a generated EPUB page directly; the next export will replace the patch.
This guide is limited to horizontal EPUB construction. Vertical combinations, right-to-left page progression, and vertical CSS are separate topics. A horizontal book is complete when direction, left-to-right progression, four navigation targets, ruby, and images agree in the same finished EPUB.
