The ImGearExcelBookDocument and ImGearExcelSheetDocument objects can be created and processed in any thread with any apartment state: STA or MTA. If they also need to be displayed, then there are some concepts which should be taken into account:
ImageGear renders only accepted revision changes of the document; i.e., if it has "Track Changes" enabled, please make sure to accept all/required changes in the document prior to opening it with ImageGear.