I searched around and I found some books on how to write proofs. There are so many of them that got good review and I have no idea which to choose. Here are some books I am considering: How to Solve It, by Polya An Introduction For Mathematical Reasoning, by Eccles The Nuts and Bolts of Proofs, by Cupillari How to Read and Do Proofs, by Solow Which one should I get? They all seem relatively expensive. Are these books much better than free online books and are they worth the price?