We sometimes get asked about sharing books via email. We haven't built this particular workflow into the app, but that's because it's very similar to how you'd email any file or link.
The main consideration here is - do you really want to send a large file over email? ePub files are often very large files, and email servers can have limits which may block the book from sending. ePub files are also essentially a zipped folder of many files, and sending by email can corrupt these files so they won't open.
So, if you want to share you book via email, here are our suggestions.
1. Publish the book and send the link
It will be easier for you to publish your book online first, and then simply copy and paste the link to the book in your email. That way, you're not actually sending a physical file, and the recipient will be able to view that book online in any browser, on any device.
2. Use a cloud service
If you do need to send the actual ePub file, rather than downloading it and attaching it to the email, why not save the file to a cloud app, and share the link via email? Please remember, when you're sharing an ePub file, the recipeint is going to need to use Book Creator or Apple Books to read the book.