Persisting Data In A Code Behind Module Between Posts

I have an ASP.NET WebForm with a code-behind module written in VBNET. I use a server component called SpreadSheetGear (SSG) that allows you to create, open, edit, calculate, and save Excel workbooks without the need for the actual Excel.exe program.  SSG essentailly takes the place of Excel.exe. SSG has no visual interface when used in ASP.NET -- everything is done programmatically.
My problem is that in the code-behind module, I create two Private variables, WB which represents an SSG.Workbook and WS which respresents an SSG.Worksheet. When the page initially loads, these variables are creaetd and an existing workbook ("Book1.xls") on the server is loaded into the WB variable.  WS is set to the worksheet "Sheet1" of WB.   I can then manipulate, calculate, etc, the workbook. SSG peforms the Excel formula calculations, so that when I change the value of one cell, the calcualted cell will reflect that new result. I then read that cell ...