The main books that are out right now are the two by Ian Roughley and this one (in print as of the 5th, I believe). Ian's first book, "Starting with Struts 2", is a great, free but short book that gives you a taste of what Struts 2 is like. His follow up book with Apress, "Practical Apache Struts 2 Web 2.0 Projects" is geared to helping you develop AJAX-heavy Web 2.0 apps using Struts 2 and as such, isn't focused on all aspects of Struts 2. Coming soon is James Holmes' book "Struts 2: The Complete Reference", which is, if his previous books are any indication, an exhaustive reference tome on the nooks and crannies of Struts 2.
Struts 2 in Action is the best book, IMHO, for learning how Struts 2 works and how it can make your applications better and developed quicker. The focus on Struts 2 broadens its appeal to the different ways it will be used, yet the practical slant ensures the material is relevant and usable in the real world.
Of course, I'm hardly the person to ask for an unbiased opinion