I have to wonder why you are doing this - are you really planning on having so many authors that you need a technical solution for this right now? Massive book publishers need that sort of thing but for a startup it seems unnecessary and a luxury. email the contracts in .pdf form, have them sign and return two copies, then mail one copy back to them.

I would just worry that you'll spend 40 hours developing a solution that will save you 5 hours a year.