ACH Processing

Does anyone know where I can get information to write software program
to do ACH processing? Ideally, I would like to be able to write a
program that customer can provide account info/routing #, etc and the
program would be able to debit the money from the customer to the
merchant. I prefer not to use a third party ACH processing vendor if
at all possible. That's why I would like to be able to write a program
that can do all of the required steps from customer providing bank
info to the Automated Clearing House (ACH) for authorization and
approval to crediting the merchant. Any help is appreciated...books
to read, seminar, example, anything. Thanks!