Is there specific file format that you have to use for the online publisher? If so, that will determine what options are available for protecting the content.
Otherwise, use Adobe Acrobat to create the book as a PDF and set the protection options to prevent printing, copying, etc.
You can't really prevent a file from being shared with other people unless you somehow link it to their specific PC/device or require online activation (similar to game activation) with a license/ID code that can only be used once. While that's possible, it seems like a lot of effort just to protect an eBook unless your publisher already has the tools in place to do it and you can just use their tools.
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