Pdf scanns of copyright-free material, usually printed before 1924.
Internet Archives - scanned books, documents, old websites all free to users.
HathiTrust - Partnership of academic & research libraries' digitized rare books
A word about scanned material. Current scanning standards for older books and material vary. Most are not scanned to high enough image quality for effective study (they are scanned primarily to be read as texts). The value of these scanning projects for image researchers is largely to divulge where valuable images are contained. Researcher frequently (and should routinely) request hard copies of these materials and scan the engravings for maps directly at a higher resolution.
Archives of American Art - files, ephemera and [non-art] collections of artists, dealers, art historians and art journalist. Much of this collection is loanable through microfilm or online as .pdf's. Separate image search.
Getty Publications free, full-text of back publications Virtual Library.