The following is the closest thing I could find to a PDF reference manual or user guide for Internet Explorer 8 (IE8). Although it describes the Beta 2 release of IE8, from a user's viewpoint, the document seems to accurately describe the final release of IE8. It contains detailed, comprehensive descriptions of not only the "whats", but also the "whys", accompanied with numerous screen snapshots.
Additionally, pages 84-86 (as labelled in the Adobe PDF Reader), or pages 81-83 (as printed in the document itself), contain a tabular feature-by-feature comparison between IE7, IE8 and Firefox 3.01.
"Internet Explorer 8 Beta 2 Evaluatorsí Guide":