One of the nice things about the Kindle is that it syncs with the Kindle app on iOS, so that you can read a few pages on your phone when you don’t have the Kindle with you.
But I thought that it only worked with books bought from Amazon and not those, for example, downloaded from Project Gutenburg. Which was annoying.
However, I have discovered that there is a way of making it work. Perhaps this is common knowledge, but I only found it by accident so I thought I’d share it.
The trick, such as it is, is to email the file to your Send-to-Kindle email address, which is the address used to add personal documents to the Kindle. It’s in the form firstname.lastname@example.org and you can find it in the Kindle settings.
Once it appears on the Kindle, it will also be available as an archived item in the Kindle app, and it should sync across devices in the normal way.
The syncing doesn’t work if you add the files to the iOS Kindle app via iTunes, or download them direct to the Kindle from the web, for example via the Project Gutenberg Magic Catalog, or if you put them onto the Kindle via USB.