How your ebook actually looks on other people's devices
You built one file. Nobody reads that file. They read what their device makes of it, and that varies more than any print run ever varies between copies.
An e-ink reader, a phone app, a tablet and a desktop app each draw your book differently — different default face, different measure, different spacing, different handling of an image wider than the column. Then the reader changes the font, sets the type two sizes larger, widens the margins and turns on dark mode, and moves it further still. All of that is permitted, and most of it is a control they paid for.
The job is not to stop the variation. It is to make sure the book works across it.
Your typeface may not be used at all
This is the hardest thing to accept for an author who has just chosen a body face for the print edition.
Many reading apps let a reader pick their own font and apply it to everything they read, and plenty of readers set that once and never think about it again. Some devices substitute a face of their own regardless. The face you embedded may be honored, ignored or overridden the moment the book opens.
So a book's identity in an ebook does not live in the body face. It lives in the structure — chapter openings, scene breaks, the shape of the contents, consistent treatment of quotes and headings — and in the cover, the one image every reader sees at full strength. Spend your design attention there, and let the body text be text.
In the Interior Formatter. It builds an EPUB 3 from your manuscript's real structure — chapter starts, scene breaks, emphasis — with a declared language, accessibility metadata and ARIA roles, so the parts that carry your book's shape survive whatever face and size the reader picks. The cover exports separately at 1600 × 2560. Start a book → · about the Interior Formatter
What to check, in order
Work down this list once, on a real device, before you upload. It takes half an hour.
- The cover at the size a store shows it. Not full screen — the thumbnail in a library grid or a search result, where a title set in fine type turns to mush. The thumbnail test is the fastest check in this whole trade, and Ebook cover size, and why it is not your print front has the dimensions.
- The contents page, entry by entry. Every entry present, in the right order, and every one landing on the chapter it names rather than the one after it. Tap all of them. A contents page that is 90% correct reads as broken.
- The first page of a chapter. Consistent space above the title, consistent handling of the number, and a first paragraph treated the same way in every chapter. This is where a structural fault shows first.
- Every image at the largest and the smallest text size. Change the reader's type size to both extremes and look again. A figure that fits at the default can force sideways scrolling at the largest setting, and a diagram with lettering in it can become unreadable at the smallest.
- A poem or a quote block. Anything whose line breaks carry meaning is the most fragile thing in a reflowable book: at a large type size the lines wrap and the shape is gone. Setting poetry without wrecking it covers how to make that wrap deliberate.
- The whole book in dark mode. See below. It is its own pass and it catches faults nothing else catches.
Dark mode is where books break
A device in dark mode does not repaint your book. It inverts the page and the text, and leaves everything else exactly as it was. Two faults follow.
An image with a white background becomes a bright rectangle. Your chapter ornament, your scanned map, your logo — each one now glows in the middle of a dark page, at night. A picture saved with a transparent background instead of a white one avoids most of this; a photograph cannot, so judge whether it is worth the flash.
Text you colored dark gray disappears. If you set a pull quote or a caption in a soft gray to make it recede on white, it recedes all the way to invisible when the page turns dark. Grays chosen against one background do not survive being flipped. Leave body text uncolored and let the device handle it.
Look at every image again in dark mode specifically, not just the ones you think of as pictures. Rules, ornaments and scene-break marks are images too on some devices.
How to check without owning six devices
You do not need a shelf of hardware.
- A phone and a desktop. The free reading apps for the major stores run on both, and between a small screen and a large one you have covered most of the ways a layout fails. The phone is compulsory: it is the smallest measure your book has to survive.
- An e-ink reader if you can borrow one. It is reflective and gray-on-gray, so anything that depended on a pale tint separates from nothing. If you cannot borrow one, convert a screenshot to grayscale and judge it that way.
- The store's own previewer. It shows that store's rendering, which is the one its customers get. Use it as well as a device, not instead of one.
Valid is not the same as right
Our EPUB 3 output carries accessibility metadata, a declared language and ARIA roles, and it validates with zero errors and zero warnings under epubcheck. That is worth having and most files do not have it — see What makes an EPUB pass validation.
It is also not what you are checking here. Validation proves the file is correct. It cannot prove the book looks right, because that depends on a screen and a set of settings no checker can see. Which is why you look.
Make yours
Everything here is built into the tools, and they are free to use: name your book, bring your manuscript or your artwork, and work as long as you like. You only pay when you want the files without the preview watermark.
Next: Checking an ebook before you publish it · More: Page numbers in an ebook, and why there are none · Accessible ebooks, and why they are now everyone's problem · Fixed layout: when the page is the design