A list from another bonsai enthusiast:
• Bonsai Today back issues, particularly the first 30.
• Principles of Bonsai Design, by Dave De Groot.
• Four Seasons of Bonsai, by Kyuzo Murata.
• Bonsai with Japanese Maples, by Peter Adams.
• Literati Style Penjing, by Zhao Qingquan.
• Classical Bonsai Art, by Bill Valavanis.
• All books by Kyosuke Gun. (in Japanese, but full of great images)