This book is very elementary, but has a lot of excellent material.

@BOOK{Polya54,
  author =       {Polya, G.},
  title =        {Mathematics and Plausible Reasoning. Vol.1: Induction and Analogy in Mathematics. Vol 2. Patterns of Plausible Inference},
  publisher =    {Princeton University Press},
  year =         {1954},
}