[dghf]

But on the other hand:

"The book belongs to the person who purchased it last week". Not "whom".

I think it is reasonable to say that the object of the to is not "who(m)ever", but the entire clause "who(m)ever purchased it last week"; and that clause should follow normal subject/verb agreement.

Similarly:

* "I don't know who purchased the book last week", not "I don't know whom purchased the book last week."

* "This is the person who you said purchased the book last week", not "This is the person whom you said purchased the book last week."

I've done some digging, and Fowler, Partridge and Gowers all support my stance, so I'm fairly confident in it now.

[Hendrikto]

Irrelevant. The original example given is in the dative case, so it has to be “whom”. It’s really as simple as that.

[dghf]

Not irrelevant at all. The case of the relative pronoun is determined by its role in the relative clause, not by the role of the relative clause in the sentence as a whole.

See:

- H. W. Fowler, A Dictionary of Modern English Usage

- Eric Partridge, Usage and Abusage

- Ernest Gowers, The Complete Plain Words

who are unanimous on this point.

[greggyb]

Perhaps you'd like to share a slightly more specific reference?

After all, these books treat many topics. Without specific reference, one might uncharitably assume that you are attempting to simultaneously misdirect and appeal to authority.

[dghf]

The Fowler and Partridge works are both arranged alphabetically, and in each the relevant entry is headed “who and whom”.

In Gowers, it is chapter 9, “The Handling of Words”, section “Troubles with Pronouns”, subsection 15, “Who and whom”; in the edition I own (Pelican Books 1962, reprinted 1970), that can be found on page 206.