On rare occasions, you will find that your Zotero bibliography in Word is missing italics, even though those are required for book titles in the citation style you've chosen. Sometimes you will find the reverse—the entire bibliography is in italics.

First, verify that the citation style in Zotero does normally use italics for book titles, e.g. by consulting the styles repository and hovering over the style in question.

If the style is displaying correctly there, you are very likely facing a known issue with Word, which will change the formatting of the entire bibliography when more than half of the first bibliography entry is in italics.

Workaround: Insert a “dummy” reference that will appear at the beginning of the bibliography and does not contain much italics (e.g a journal article by Abu Aardvark). Obviously you can delete that reference as the last step of editing.