The first book in http://www.amazon.com/s/ref=sr_nr_n_0?fst=as%3Aoff&rh=n%3A283155%2Cn%3A2691%2Ck%3Amrp&keywords=mrp&ie=UTF8&qid=1461053617&rnid=1000 seems useful as a start of your studies about MRP.

Once you understands what it is, have a look at https://www.google.com/search?q=open+source+mrp+software.

When writing a requirements document, it's essential to involve the future users of the system. Most likely they are the current users of whatever is used now (either an MRP system that has to be replace, or a manual or spreadsheet solution),
Writing a requirements document without understanding the workflow that has to be planned, is impossible. And that workflow is NOT in the books and NOT in the software you can study. It's in the heads of the people that do it now,

Kees