Well, the author could best answer this, but being a "Complete Reference" I would think all sorts of people could get benefit from it, but it might be better suited for those who have some knowledge of Struts.
As for a beginner book, try "The Struts Framework" by Speilman. It is a small book that gets right to the point without a lot of fluff. I like it and have communicated with the author several times.
The only thing I wish was covered and wasn't is Struts to
EJB connectivity.