Book View when you hit Enter will repeat whatever tags exist on hte block you edited.
So if you start off with text in <div> tags, and then hit enter to type more text, it too will end up in div tags with the same class applied if any.
If you want text in <p> tags in Book View, just make sure you start you editing within <p> tags (which is what Sigil does by default). That it "went wrong" for you simply means that your text you hit enter within originally was a <div> rather than a <p> tag.
Note that in Book View you can also ensure the current text block is within <p> tags by using the Headings toolbar/Format menu option, to choose the "Normal" option. That will switch any block tag (headings, blockquotes, or divs etc) into a <p>.