I write a chapter for my book in logic concerning Gentzen systems and natural deduction. I read the paper of Gentzen and I realized that his motivation was to create a system that works as close as to human reasoning .... In my current book version, I present first Gentzen systems, and then the natural deduction, but after all this info I wonder if I have to present natural deduction first. What is your opinion about this? If there are some classical references I don't know, please let me know. Thanks a lot !!