Following is a list of some titles that I've found beautiful or useful or both. I'm not gong to say much about them here, but they're all likely to give you interesting things to think about.

• Introduction to Graph Theory (Richard Trudeau)
  
• The Art of Computer Programming (Donald E. Knuth)
  
• What Is the Name of This Book (Raymond Smullyan)
  
• The Little Schemer (Friedmann & Felleisen)
  
• Gödel, Escher, Bach (Douglas Hofstadter)
  
• Automata Theory, Languages and Computation (Hopcroft, Motwani, and Ullman)
  
• The Mathematics of Life (Ian Stewart)
  
• Gödel's Proof (Nagel & Newman)
  
• Programming Pearls (Jon Bentley)
  
• Introduction to Number Theory (George Andrews)