Hello Helmut,
I can recommend a couple of fantastic Excel books, specifically about Excel reporting features. There's nothing specific to Progress (almost), although, to use a SQL data source you're going need a fairly recent version, 10.1x.
Excel can be a very powerful, easy to use, easy to setup solution and users take to it right away. A lot of people and managers are surprised when they get to know Excel's reporting features. Pivot Tables and Pivot Charts, generate off line OLAP cubes, Web enabled reports etc.
If you need help setting up or you'd like to know more about some of the Excel features and hear some of the experiences we've had, feel free to contact me privately (though, I'm moving to a new apartment over the weekend and may be unreachable for a number of days).