There is indeed an API. Start here: http://doc.openerp.com/v6.1/developer/12_api.html

You may get some useful ideas from a project we are authoring:

Google Spreadsheets to OpenERP Data Pump

Mainly, it binds third party modules, so the actual amount of code to look at is very small.

While still in an early beta-stage of development, we have tested it for :

• instantiating a fresh database
• installing modules
• updating existing data
• loading data in bulk.

The key benefit is that you can declare what you want done in a Google spreadsheet, as long as a class and method to service that task exist. Meanwhile, new classes and methods require very little code.