JasperSoft provides online training for JasperReports and the iReport designer. You can find it here: https://www.jaspersoft.com/bi-training-center
Some of the content is free, some of the content is being charged for with a monthly subscription around $200 per month.
One free nugget you should propably have a look at is the configuration of the iReport Table Component.
If that link won't work over time any more, this is how I got there: Home > Services > Training > Online Learning Portal > Enabling Report Interactivity with the Table Component.
Why is that important? In order to have an additional detail query in your report you would have to create a subreport to achieve that. Using subreports requires you to use additional files for each detail query.
Using the Table Component is an alternative approach to that being able to use a subquery within the current report definition - no longer having to use a separate file.
Although this feature is around for a long time (I guess for a year or two) it is not well documented. So you should definitely have a look.
Update 12.07.2012: Here is another link: https://www.jaspersoft.com/blog-entry/jasperreports-interactivity-the-table-component
I have also updated the link for the configuration of the iReport Table Component, it was broken.