During the time that the Coursera Devlin Mathematical Thinking course is running in September and October 2012, I will lower the price on my Peano Axioms ebook to 2.99 if ten people will subscribe their real names here (or email me instead) and what country they live in and promise to review the book at the e-vendor they purchase it from. They also should email me in any case. This is a non-binding offer until I receive ten names in emails that I consider valid.
The Peano Axiom book has proofs using mathematical induction that don’t involve complicated algebra formulas. Lance Rips et al have shown students most complain about the algebra formulas in learning mathematical induction proofs.