Dover's collection of clip art books has seen some improvements over the years. They now almost exclusively include a CD of all of the images so they are ready for use. Some of my older Dover books do not include CDs, making their use a bit tedious. Dover also began doing full color books for certain publications.
These are some of my favorites for black and white line drawings, hotlinked to Dover's site.
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Dover also carries a line of books with full color and vector images that can be resized. The Pictura series are extremely detailed and useful for even large scale projects.
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Pictura includes books of design and art from most of the world's cultures and detailed books of buterflies, flowers, insects and various art styles.