Pardon the stupid question, I've been out of school for some years now and no longer actively involved in history other than self-study. Would a modern biography be considered a secondary or tertiary source for the purpose of this subreddit?
Example - Anthony Everitt's biographies of Cicero and Augustus cite both primary sources, as well as secondary sources. Secondary, or tertiary?
That's a secondary source. Any academic book or article with original work will cite other people's work, and I would be highly suspicious of any book that didn't cite anyone else in the field and just cited primary sources, I'd wonder if they did any reading at all!