Record the source and rights holder for every substantial quotation, image or other material you did not create. Rules vary by country and context, so do not assume that attribution alone grants permission. Short excerpts may still require review, especially for lyrics and unpublished work.
Ask your editor or publishing professional to flag material that may need clearance, then seek appropriate legal guidance when necessary. Starting early gives you time to request permission, pay fees if required or replace material before design and production are complete.