Sometimes Zotero will import a citation with the Title or Publication (journal title) fields in ALL CAPS. The ALL CAPS will be transferred to your bibliography, so you'll want to change it.
Often you want to cite a book chapter instead of the entire book, especially if it is a chapter from an edited book. You can easily change a book record in Zotero to a chapter.
Don't forget that grammatical rules change from language to language, particularly rules around capitalization.
If you're doing research in multiple languages and your citations are incorrectly capitalized, try specifying which language the item is in so that the citation style can apply punctuation and capitalization rules appropriately (if this doesn't work, you can always manually change the entry as described above).