edit: I apologize for not saying this more artfully but what you ask is not a straightforward question with a binary choice.

These questions must be answered:

Is the "book" a distinct published item with its own metadata from the complete
"conference paper" that was published elsewhere, perhaps only an abstract?

(Is the publisher the same for both items?)

Also, Is this item the entire book or is it a part of a collection of conference papers in a conference proceedings (and thus a Book Section)?

Is everything except the item type exactly the same? ~Did you, in your search, capture the same thing from two online sources with one labeling the item as a conference paper while the other a book? (I doubt that the item is likely to be a full book.)

If not exactly the same in your Zotero records, are the items actually different representations of the same material? [edit: if different representations, they are not "duplicates".

Are both items representations with the same degree of completeness of the text? An alternative is to keep both items but use only one as your citation. Cite the version that the readers of your manuscript will more easily find.]

These are questions that must be answered when weighing whether to delete one or merge. If you want to merge, and the metadata is essentially the same; pick the most appropriate item type and change the other's type so that it matches. [Book type is probably wrong if the item is from a proceeding book and should be a book section type. Book and book section types require different levels of metadata completeness and must be cited differently in your manuscript.]

@DWL-SDCA and @tim820 , thank you very much for the detailed replies!