Q. In Spanish text, titles of books are set in sentence case, but should they also be set in italics? And how about titles of book series and the like?
A. If you’re applying Chicago style, and often even if you’re not, book titles are italicized in Spanish-language contexts, whereas series titles are usually in roman (regular) type—both as they would be in English.
So, whether your text is in Spanish or English, you would refer to The Chicago Manual of Style (note the title case, usually retained for English titles in Spanish-language contexts) but el (or “the” in English contexts) Manual de estilo Chicago-Deusto (a version of CMOS adapted for Spanish and published in 2013 by the University of Deusto, or Universidad de Deusto, in partnership with the University of Chicago Press; as with books in English, an initial article isn’t always part of the title).
As for a book series, you would refer to (for example) Bonsái (Editorial Anagrama, 2006), a novel by Alejandro Zambra published as part of the series Narrativas hispánicas. Again, these novel and series titles would be treated the same whether your text is in English or Spanish. Titles of series, though, occasionally appear in title case, which is how names of periodicals and publishers are usually treated (e.g., the journal Revista Mexicana de Sociología or the publisher Fondo de Cultura Económica). When in doubt, follow the usage in the source.
For more guidance, consult Chicago-Deusto. Additional resources are available online from the Real Academia Española, including the freely accessible Libro de estilo de la lengua española (a Spanish stylebook first published in 2018); in that guide, see “Clase de letra” (on styling text), and especially “Cursiva demarcativa para delimitar expresiones denominativas” (on italics for titles of books and the like).
In CMOS, see paragraphs 11.67–76. For the titles of book series, which are sometimes in italics, see 8.178.