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OK, nobody wants to hear this, but that's horseshit, not grammar. Grammar Girl on who/whom:
Use whom when you are referring to the object of a sentence.
For example, it is, "Whom did you step on?" if you were trying to figure out that I had squished Squiggly*. Similarly, it would be, "Whom do I love?" because you are asking about the object — the target of my love.
Horseshit. "Whom" is used today, if at all, only after prepositions: "To whom am I speaking?" To say "Whom did you step on?" or (god forbid) "Whom do I love?" is to mark yourself not as a supergrammarian but as a hopeless twit. I long for the day when people will be confident enough of their grasp of their own language that they won't waste their time (and in the worst cases money) on quacks like this. If you want serious advice, backed up by actual research, I recommend (as always) the Merriam-Webster Dictionary of English Usage.
Heh. Well, the LibraryThing page has links (in the left-hand column) to Abebooks, Alibris, Amazon.com, Barnes & Noble, and BookFinder.com, among others; here's the page for the original book, and here's the Concise version (which is slightly abridged but also more up-to-date, so probably the better buy).