I would like to embed a Google spreadsheet directly into my email, so that it reflects the document changes. How do I do that?
Not sure how you could embed the actual spreadsheet, but you can at least embed a link!
You can capture the link as a web page: File > Publish to the Web... > Capture the link from there to embed in your email.
or - for more granular security control - you can explore the Share feature and capture the link from there.