Some ebooks use proprietary formats, like the Kindle. Amazon has a converter that takes your electronic file and [del]butchers it[/del] converts it to its format. There are several other proprietary formats as well. Kindle won’t convert from pdf’s because they don’t scale the way a font does, although programs are available to allow you to read a pdf on a Kindle.
Other readers can use .pdf files or .doc files or .txt files or any one or more a dozen others.
There are sites and firms that will convert books for you in these various formats for reading on the various devices.
Books with straight text are fairly easy to convert. Books with many fonts and formats are harder. Images are a problem unless embedded in a .pdf file. They may or may not transfer and may or may not appear where they are supposed to. Footnotes also have problems. Sometimes footnotes are clickable, sometimes they aren’t. Kindle converts footnotes to endnotes, which can really screw things up.
IOW, there are dozens of readers, dozens of formats, dozens of converters, dozens of original forms, and millions of ways that it can be done and can be screwed up. What works on one reader may not work at all on another. It’s a field in its infancy and will be cited in the future as an object lesson in why standards are so important.