It is safe NOT to sure undrm'ed ebooks

EPUB internally uses XHTML or XML to represent the text and structure of the content document, and a subset of CSS to provide layout and formatting. XML is used to create the document manifest, table of contents, and EPUB metadata. Finally, the files are bundled in a zip file as a packaging format.

So you can rename an .EPUB file into .ZIP file, extract its content and see for yourself using any text editor.