ASP.NET AJAX control toolkit is a great set of helpful controls for ASP.NET AJAX. Usually many of these controls aren't covered in books and articles most likely because their usage is simple.
All you need to use this control is a ScriptManager control on your page. After that, you can insert this control somewhere in your page. Obviously you need to use this control with TextBox controls that have their TextMode property set to Password.
This control has several properties that customize its behavior but many of them are optional and control uses their default values. The only property that you need to take care about is that TargetControlID which is required and must be set to the name of your TextBox control.