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