Which is okay in conversation, but in writing, your pedantic editor or proofreader would probably inform you that it is considered poor style to end the sentence on a preposition. So he's basically trading one error for another.

Who/whom is indeed awkward at first to remember, but the phrase "for whom" tends to sound better on the tongue than some of the others due to its inclusion in the title "For Whom the Bell Tolls," a popular novel (or "popular Metallica song", depending on your age).

> it is considered poor style to end the sentence on a preposition

By some, sure, but not really legitimately. See https://en.wikipedia.org/wiki/Preposition_stranding#Opinions..., and of course plenty on Language Log including http://languagelog.ldc.upenn.edu/nll/?p=2752

Which would also be incorrect.

You're downvoted but you're not wrong. 'Whom is this book for?' is correct.

You can tell because the answer is 'her/him', not 'he/she'.

See: http://www.grammarbook.com/grammar/whoVwhom.asp

Incorrect according to whom?

If you can understand it, it's not incorrect. You might not like the style, but that doesn't make it wrong.

> If you can understand it, it's not incorrect.

acn yuo stunderand htis? i no oyu cna so acocrding ot yuo its nto rwong. oaky.

That's clearly not what I meant. Grammatical prescriptivism is considered incorrect by pretty much any linguist.

Given the rephrasing was done to avoid the question of "should we use 'who' or 'whom' here?", I think it's fair to point out that the new phrasing suffers from the same problem.