If you type "Design" into the Gutenberg search engine a few useful books, not all directly to do with leatherwork, come up.
e.g
www.gutenberg.org/ebooks/22107 Wood Carving by George Jack
www.gutenberg.org/ebooks/38187 Aviation Engines both useful source material for carving, steampunk etc.
www.gutenberg.org/ebooks/16273/ Manual of Heraldry
www.gutenberg.org/ebooks/139749/ Principles of Decorative Design
www.gutenberg.org/ebooks/42951 A new century of Inventions
and many more