I'm reading "JasperReports 3.5 for Java Developers" and have reached chapter 10 about iReport.
Is iReport relevant if you're using Jaspersoft Studio?
I think it is.
I also have been using this book next to 'The definitive guide to iReport'. After some days/weeks working with jasperreports I stumbled upon the JasperSoft Studio project. As far as I discovered, this tool will replace or at least make the iReports tool irrelevant.
However, I have the impression there are still some bugs in JasperSoft studio. I also find it hard to find proper documentation for it. Some iReports documentation is still relevant though.
That's just my two cents. The people of JasperForge should be able to give a more clear answer.