>>Take a look at the Demo() method -- it has example calls for creating a multi-sheet workbook. The problem you came across is due to creating a workbook with a sheet and not assigning any text or values to any of the cells. In this case, since there is not any text assigned, the internal strings.xml is not needed and is removed. But I had an error in the logic that removes the string.xml from being written which causes the sheet to not be written. I have actually fixed this issue and will be uploading a new beta version (probably tomorrow).
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>Along the same lines, I tried creating multiple worksheets (15) in a new workbook in a loop. When each was left blank, only the first 2 sheets were created. However, if as part of the sheet creation process I added a value to a cell on that sheet, all 15 sheets were created.

Have you updated to the latest beta version? I uploaded it this morning (Friday, May 7) which should fix the issue above.