Sigil simplifies creating and editing EPUBs

Creating an ebook in EPUB format is easy, for certain values of "easy". All one really needs is a text editor, a few command-line utilities; also needed is a working knowledge of XHTML, CSS, along with an understanding of the format's structure and required boilerplate. Creating a well-formatted and attractive ebook is a bit harder. However, it can be made easier with an application custom-made for the purpose. Sigil is an EPUB editor that provides the tooling authors and publishers may be looking for.

About Sigil

Sigil is basically a one-stop shop for creating and working with publications in EPUB format; it supports both the EPUB 2 and EPUB 3 standards. Users will generally not want to create new publications using EPUB 2, since it was superseded by EPUB 3 in 2011, but version 2 support may be useful when working with legacy publications or creating ebooks for people who may not have up-to-date ebook readers.

It is a multi-platform desktop application that uses the Qt 6 framework and is written primarily in C++, with a fair amount of C and Python as well. The GPLv3-licensed project got its start in 2009, when Strahinja Marković began working on it as part of his computer science course work.

Since then, Sigil has had a few periods of inactivity, one of which led to Kovid Goyal to create an ebook editor for the Calibre project in 2013, when it appeared that Sigil had gone dormant for good. Work on Sigil resumed in 2014 when Kevin Hendricks and Doug Massay took over maintenance of the project, which is hosted on GitHub. The pair have continued to work on Sigil since then; the project is actively maintained and has published eight releases over the course of 2025. The application is fairly mature at this point, so most of the work consists of minor new features, bug fixes, and updates to keep up with changing dependencies.

Documentation

The user guide for Sigil is provided as an EPUB, of course. There is also a version of the user guide that is readable online; it is the same EPUB content but served by Readium, a project for displaying EPUB content in web browsers.

It is always a good idea to start with a project's documentation, but it's an especially good idea with Sigil if one doesn't have much experience tinkering with EPUBs. The guide not only provides a reference to Sigil's features, it has a number of short tutorials as well. For example, it has useful guidance on converting content from LibreOffice (ODF) or Microsoft Word (DOCX) formats into XHTML, as Sigil does not import those formats directly.

I'd recommend starting with the tutorials and then doubling back and skimming through the "Features" section, which serves as a good reference of Sigil's interface, tools, and capabilities. The user guide EPUB is also useful as a test document for working with Sigil. It provides a good example of an EPUB's structure, well-formatted CSS and XHTML, and other metadata documents.

Using Sigil

On first glance, Sigil looks like a basic text editor with syntax highlighting for code—which it is, but there's a bit more to it than that. The default layout includes Sigil's menu bar and a number of toolbars at the top of the application window. Below the toolbars, Sigil displays the "Book Browser" pane on the left-hand side, a text editor in the middle, and a preview pane on the right-hand side that shows what the final page layout would look like. That is, assuming that a reader's ebook reader renders an EPUB's styles the same way that Sigil does, which is difficult to predict.

Users can hide or move the various toolbars and preview panes, except for the middle text-editor pane. The panes can also be popped out of the main window into a separate floating window.

It's worth noting that Sigil is not a good choice for actually writing content or collaborating on a publication before it is ready to be turned into an EPUB. Sigil is designed for assembling content into a book, or massaging content that is already in EPUB format. The application's editor is not really meant for authoring or heavy copy editing of content; it is not a pleasant writing environment or particularly ergonomic for editing prose. It is really only suitable for light editing and adding markup to the text.

That is not to imply that Sigil does not have any tools to assist with copy editing, though; it has a decent spell checker, and supports Perl-compatible regular expressions (PCRE) for searching and replacing text via the PCRE2 library. There is also a "Saved Searches" utility for users to create and save search-and-replace operations that are used often. For example, Sigil comes with a number of example searches for removing empty paragraphs, trailing spaces, and for converting characters like the em dash ("—") to XHTML entities ("—").

The project does provide a separate XHTML editor called PageEdit, which allows users to create content in a WYSIWYG editor. It is much more suitable for writing and laying out a chapter of a book or similar before importing it into Sigil for assembly into a final publication. Users can also use the external editor of their choice with Sigil; those who prefer to use Emacs, nano, Vim or others can do so. PageEdit may also be worth a look for those who would like a basic WYSIWYG HTML editor for other purposes.

What Sigil is best at is assembling content into EPUB format, refining it to be more usable for the reader and checking the results to ensure that the publication conforms to the EPUB standard (though a plugin is required for version 3). It is not terribly difficult to generate XHTML files for chapters using one's favorite text editor or word processor, for example, but creating a table of contents or index is another story. Happily, Sigil has easy-to-use tools for doing both.

Sigil can generate a table of contents from headings in a book, so each heading tag (<h1> through <h4>) becomes an entry in the table of contents. If that is not quite what is desired—perhaps including <h3> and <h4> headings would result in too many entries—users can choose to only include headings of a certain level. It's also possible to edit the table manually and add or remove entries.

Creating an index is a more manual process, but still much easier with Sigil's help. Users can mark any text, such as a person's name or references to specific programs, and select it to be included in the index. Alternatively, a user can include all instances of a word or text string in the index by adding it in Sigil's "Index Editor" tool. It's best to reserve that for rarer words or names, though. Once all of the text is marked up or added via the editor to be included in the publication's index, Sigil can then automatically create an index that refers back to each entry. This can be recreated as often as needed.

Plugins and automation

Sigil supports plugins, written in Python, to extend its functionality. There is an API guide (also available as an EPUB) that includes information on the expected structure of a plugin, and an example plugin.

It may not be necessary to write a plugin, however. There are many plugins already that provide features that might be wanted, such as importing DOCX files, validating an EPUB using the WC3's EPUBCheck utility, and improving an EPUB's accessibility features. There is an index of plugins available on the MobileRead forum. The EPUBCheck plugin, in particular, is something users should add immediately after installing Sigil to identify errors and help ensure that an EPUB conforms to the standard.

If one is processing a lot of publications with Sigil, it's likely that there will be combinations of operations that are ripe for automation; naturally, Sigil has a tool for that as well. Users can create automations that consist of combinations of tool operations (e.g., deleting unused CSS styles) and plugins.

One of the features I most appreciate is checkpoints; a checkpoint is a snapshot of an EPUB at a point in time. A user can create a checkpoint, work on their EPUB, and then compare the current state to the prior checkpoints to see what has changed. Sigil will show what files have been added or deleted, and what files have been changed. Users can also inspect the diffs of changes in a file between checkpoints. Best of all, users can restore from previous checkpoints if needed—so if a file has been accidentally deleted, or a bunch of changes need to be reverted, checkpoints can be quite useful. They are, however, an all-or-nothing affair. Sigil doesn't support, for instance, just restoring one set of edits or one file from a checkpoint if multiple files have been deleted. And, as Sigil will warn, restoring a checkpoint overwrites the current status of a project—so it's a good idea to take checkpoints often and save one's work in a new EPUB before restoring from a checkpoint.

The most recent release, version 2.7.0, improved navigation in Sigil's metadata editor, updated the menu for user-created automated actions, and included a number of bug fixes. The Sigil project provides builds for Linux in the AppImage format with each release. The project also points users to Flathub for a Flatpak build, but it apparently has not confirmed ownership of the Flatpak, so it shows up as an unverified app. Most major Linux distributions provide Sigil packages as well.

The Sigil project's support community gathers on the MobileRead forum; the project developers seem fairly active there. Users can report bugs and other problems using GitHub Issues.

Generally, I've found Sigil easy to work with, if a bit clunky in places. As an example, when using the table-of-contents (TOC) editor, it is not possible to select multiple entries at once to change the heading level or remove them from the TOC entirely; it can only be done one at a time. But, a few minor gripes aside, it is a suitable tool for making and working with EPUBs.