The Symbian S60 platform is used by Nokia, Samsung, and many other manufacturers, and you'll find S60 smartphones in the hands of many people these days. Now all of those people can read and write Open XML documents on their phones. You can send a DOCX or an XLSX to a colleague with a Nokia 5500 Sport like the one shown here, and they can edit it and send it back.
There probably won't be many books written on these devices, but I'll bet quite a few busy managers will browse XLSX spreadsheets on them. Dataviz's Documents to Go offers similar read/edit/create capabilities for the PalmOS (they support PPTX files too), and the iPhone offers Open XML support too.