In 2011 the IIARF published Delivering Audit Reports that Matter by Sally F. Cutler. It's an exellent resource for internal audit reporting best practices and example reports. It's available in the IIA Book Store.
I f you haven't done so already, you may want to research to the digital versions of Internal Auditor for the last few years for articles on report writing best practices. I recall at least one (it may have been in the "Back to Basics" section or one of the columnists - Jacka??).
If you have access, the IIA's AEC issued a Knowledge Report Audit Report Writing Essentials in August 2011.
Edited: Tuesday January 15, 2013 at 1:24 PM by Mark R. Simmons