Google Art Project. Click on "Collections", "Artists" or "Artworks", pick a work, and zoom in to your heart's content!
Hundreds of free pdfs from the Metropolitan Museum of Art, including major catalaogues of Vermeer, Velasquez, Ingres, Seurat, and many others.
Similar collection of free pdfs from the Getty Institute
Good online source of technical information on oil painting media, permanence, etc:
James Gurney's blog on painting and colour
Two original texts on painting in layers:
Bardwell, Thomas, 1756. The practice of painting and perspective made easy.(flesh painting pp. 13-21, draperies pp. 23-34).
Templeton, 1845. The guide to oil painting (see "Methods of applying the colours", pp. 36-67).