Secondary literature on a wide variety of disciplines, and largely but not exclusively academic (read: peer-reviewed). Also includes news, newspaper, trade, and popular publications. Coverage highly variable both for individual titles and available publication dates.
JSTOR collections I-III. Please note that JSTOR is an archive, and is devoted to largely academic journal content. JSTOR does contain some interesting primary source material, e.g., pamphlets and broadsides, and the database offers full-text access. It is only searchable by keyword, and many entries lack abstracts and indexing.