[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.