When I first started leather working, much of what I learned about wallet design was gleaned from the videos of Steve Montelli. Here is a recent video he did in review of two wallets that I recently sent his way. Since I am just getting started in leather work, my plan is to send him a few more wallets this winter once my skills have improved.
https://www.youtube.com/watch?v=J4n4ZiTFx3o