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Calibre and epub works very well.
You can set it so that calibre will automatically convert the book to the correct format and email it to your kindle (you need to also set that up on your kindle account). That means that it will be present on all your kindle readers (actual kindle or phone/tablet) and synchronised to the last place read.
You sometimes need to adjust line spacing or font size on the kindle as different files look different but it's normally pretty good.
Thanks!
I'll be using epub for library and Gutenberg books.