Putting the approvers in the session will work also. However, when you're working with popup windows, there's no way to avoid Javascript entirely. You must use it to open and close the window.
If you're going to submit a form from the popup window, here's what I'd recommend: Create a small HTML file named closeWindow.htm that contains the following:
*Note onnload should be onload
Then have your Action class forward to "closeWindow.htm" to close the window after it has put the data in the session. The reason I recommend doing it this way is that when I've tried to close the window in the JSP that contains the form, I've run into strange problems with the fact that sometimes the window closes before the form gets properly submitted and the data is lost.
In answer to your question about reference material, I've found that one of the best sites is Microsoft Development Network (MSDN). Obviously it assumes you're using Microsoft Internet Explorer, so you have to be careful to cross-reference it with other sources to make sure what it tells you is valid for other browsers. Here is the link:
http://msdn2.microsoft.com/en-us/library/aa342502.aspx