EPUB Merge / Split
Combine EPUBs, split one by chapter, or edit title, author and cover art — no ads, no account, no upload. Calibre is heavyweight for a one-off; the online converters are ad farms. This is neither.
Add two or more EPUBs, put them in the order you want, and merge them into a single book with one working table of contents. Chapters, images, fonts and stylesheets all come along — each source book gets its own namespaced folder inside the merged file so nothing collides.
How this works & why it’s private
An EPUB is just a zip file with a required structure: a META-INF/container.xml pointer, an OPF file listing every resource (the manifest) and their reading order (the spine), plus the XHTML chapters, stylesheets, images and a table of contents (nav.xhtml for EPUB3, toc.ncx for EPUB2). Everything here — reading the zip, rewriting the manifest and spine, regenerating the table of contents, editing metadata, and writing a new zip — happens with JSZip and the browser's built-in XML parser, in memory, in this tab.
Merge puts every source book's files into its own namespaced folder (b0/, b1/, …) so filenames never collide, concatenates the spines in your chosen order, and builds a fresh two-level table of contents. Split copies the full resource set (images, fonts, CSS) into each output so every file is self-contained, even though most files only use a fraction of it — that trade-off keeps splitting reliable instead of chasing every cross-reference. Edit metadata rewrites the dc:title/dc:creator elements in the OPF and swaps the cover file's bytes in place.
To verify none of this touches a network: open your browser's dev-tools Network tab, then merge, split or edit a book, and watch the live "uploads" counter in the nav above — it stays at 0 the whole time.