Some XLS file (e.g. the ones created by Borlands Crystal tool) do contain all upper case directory node entries, e.g. "WORKBOOK" instead of "Workbook". These files can't be read with POI. It would make sense to allow for both forms.
It's not impossible that this change could be made Could you please upload one of these files, so we'll have something to test against if we do go ahead with the change?
Created attachment 19058 [details] An example with "WORKBOOK" in upper captial. This is an example of a XLS file which can not be read with POI due to the upper / lower case issue described in the bug report.
I've committed a patch to HSSFWorkbook, which allows it to open both files containing Workbook, and also WORKBOOK If you do a svn checkout, or get tomorrow's nightly build, hopefully your problem excel files will now be openable.