I’ve always loved the Dover Clip-Art Series Books. You can still find them at your local used book store or at on-line book stores like Abe.com. (external link).
I bought three today: Advertising Art in The Art Deco Style, Art Deco Borders, both by Theodore Menten, and Art Nouveau Animal Designs and Patterns by M.P. Verneuil.
If you love clip art, line art, black and white illustrations, stenciling, or are just interested in Deco and Nouveau, order a few of these Dover Books. You’ll be very happy.