Sometimes older books have a smell that I would consider a dusty smell. Not a sour smell or unpleasant for that matter just dusty. Is that the same thing as musty? When I think of musty I think of the smell when you leave wet laundry lying around for too long. Mildewy is probably another way to put it.

I think my definition of musty would  be more along the lines of a moldy/sour odor.   I happen to like that 'dusty' old book smell, and I don't think that dusty smell can be helped if the book is kept in a room that gets little activity.  My books from my office smell different that my books from my living room!

Musty is a bad smell.  Dusty is just a smell.

Musty is the inside of a damp towel left in a pile on the floor, a wet basement. rotting leaves, gym clothes not aired out.  Dusty gets better if you air out the book. Musty generally does not get better.

I know that I get the "dusty" smell on some of my books. I have so many that some were packed in boxes... You can tell the difference between Musty and Dusty when you come across one.. The smell is very different!!

To me, musty is the smell of your aging great-aunt's house or your grandmother's where things have been packed away for a long, long time, maybe with mothballs, maybe not.

Moldy would be the smell of the wet clothes in the laundry bag or a heap of wet towels laying around until the odor makes you notice them.  If your book smells like this, I'd think it would also have a very high likelihood that it also has water damage.

Dusty would just make me sneeze.

Lol - well, I had to look this word up, because to me, "musty" is just that old packed away in an attic or trunk smell, and doesn't have a necessary association with mold & mildew - which if you've ever encountered that in towels or clothing that were left out wet, has an unlovely odor all its own...  Dusty, I just associate with sneezing.

In terms of just a generalized google search, it appears to be about 50/50 - some calling it a musty old attic or closet odor, and others that unequivocally associate it with mold and/or mildew.  In literal terms, it IS unequivocally associated with mold and/or mildew:

mus·ty
–adjective, -ti·er, -ti·est.

1. having an odor or flavor suggestive of mold, as old buildings, long-closed rooms, or stale food.                            2. obsolete; outdated; antiquated: musty laws.
3. dull; apathetic. 
                             

Online Etymology Dictionary :  musty - adjective
1. covered with or smelling of mold; "moldy bread"; "a moldy (or musty) odor" [syn: moldy]
2. stale and unclean smelling

This is why I had to deny requestor conditions on a book request I got. They had allergies and didn't want books with a "musty" smell. Well my book didn't smell musty as far as I was concerned but possibly could have had a dusty smell that I couldn't detect since I have a cold. I figured dust could set off allergies as well. Anyway, better safe than sorry I suppose.

Thanks for all of your input.

I can see your point; my own sense of smell is sometimes fluky due to nasal allergies, so it's not always 100% perfect.  I'd err on the side of caution if in doubt.  Aside from which, this one - this particular term - I think has a lot of current & potential misunderstandings associated with it.  Dictionary terms notwithstanding, my definition of musty still doesn't exclusively translate as moldy or mildewy odors.  But I know what those odors are; I have kids and they leave everything outside to get wet.:P   And if something smells like mold or mildew - a damp, fungal smell - and you can't bleach it (I hope nobody tries to bleach a book - lol), it needs to go in the trash and DEFINITELY not end up posted on a PBS bookshelf.  I know if a book smells like that, it's at least been exposed to mold or mildew, and can't be passed on to someone else.  Musty, IMO, is the general odor I detect whenever I go into a thrift store, or the little cubbyhole sized UBS.  That is the odor that can be removed from used books - in otherwise good condition - by storing it for a couple of days in an airtight plastic tote with baking soda and cat litter.  There isn't enough baking soda in the world to absorb the odors of mold & mildew, and I wouldn't even waste my time trying.  But again, I don't think the differences are unequivocal & universal to most people, so if someone tells me their books are dusty, I'm gonna say dust 'em.  If they say musty, I'll recommend ways to deodorize them, and if they say moldy, I say throw them out before it spreads and ruins all of your books.  But somebody else?  Who knows?

I declined a request recently for the same reason.  I didn't think the book smelled at all, but it was a little too subjective for my comfort.

If i ever see a musty rq again i am declining. Its to subjective andi got aRWP from two older books i sent that to me simply smelled like older books not musty.