[greggyb]

Yet "whomever" is also the object of the preposition, "to".

Certainly, if we took the primary clause of the sentence and substite in any number of pronouns, you'd agree that the objective forms are correct:

The book belongs to whomever. Not "whoever".

The book belongs to her. Not "she".

The book belongs to us. Not "we".

I don't know the English grammatical rule for this situation, but it certainly seems reasonable to say that the dependent clause does not get to dictate the form of an independent clause.

[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.