[Echoing Others' Thoughts]

In spite of its convenience, I don't believe that The Presence of Others is worth our students' money--at least, not in my classes.  The only unit during which I actually utilize the book is with the critical analysis.  I could just as easily hunt down an independent article to have my students discuss, and they wouldn't have to dish out money for a fourth book that fails to inspire me.  Personally, I think the other three books cover the basics quite effectively.

I also want to echo Sarah's sentiments on the Hacker manual.  I, too, suggest the Prentice Hall Reference Guide, by Muriel Harris; the explanations are more in-depth, each rule is carefully described, and the book offers plenty of samples to help illustrate how to cite these references.  Since receiving and comparing the two books (on assignments for my own classes), I made the decision to have my students return the Hacker manual and buy the Harris one instead this semester.  It's easier to navigate, and I get fewer of the citation questions because they understand it better.

Also, the grammar sections offer great layman's terms for grammatical rules.  I definitely feel like the grammar rules that once perplexed me now make more sense.  The exercises also help to reinforce those habits.  

I have no alternative book suggestion for The Presence of Others, mostly because, in my class, I don't see the need for four books.  Anyway, that's my two cents.