Kunst-Formen der Natur (Art Forms in Nature), by Ernst Haeckel, 1898. This is a wonderful book, in the public domain, which features all sorts of illustrations of the natural world. If you're doing some research for organic shapes, this book is a nice place to start.

I've uploaded a 260MB PDF version of this 400 page document to my website, please feel free to download!

Update: I found the original source of this PDF, located here - much thanks to Kurt Stüber for scanning this fantastic public domain book!