Discussion Forum—A Way with Words, a fun radio show and podcast about language

I find myself running into people recently who must have been taught from a different grammar book than I was. These people insist upon "correcting" sentences like "Harry is the only mechanic that can save your engine now", changing that to who.

I was told by my ninth-grade English teacher in no uncertain terms: "who is for people, which is for things, that is for either". Is there really a straightforward rule that governs these things, and if so what is that rule? Or is there only an arcane set of special cases and exceptions?

Probably related to this question, how is it that a form of who is used even for inanimate objects when the possessive form is called for? I get the difference between who and whom, and that the corresponding case for which or that uses the same form for subject and object, but what's the deal with "I just read a book whose pages were stained by the tears of the previous owner"? Wouldn't logic dictate something like "a book which's pages"? You certainly wouldn't say "It's a book who my chiropractor recommended".