How to get running YTD totals from previous workbook into new workbook

[mkater]

I created a workbook with several sheets. I create a new workbook each week from a template. The 1st sheet is a company summary of all the employees data. Each following worksheet is named after each employee and has weekly data summary for that specific employee in various topics. For example, number of sales, number of appointments, ect....Each sheet has totals for that week. I would like to have a running YTD total and average, which means that the current workbook needs to look back at the previous workbook. What I would like to do is to find a way for the new workbook to look back at the previous one to get a continueous YTD total and YTD averages without changing every single formula in the new workbook. Is there a way I can do this?