I ordered a huge book. Hardcover, 650+ pages. The sender mailed it in a brown paper kraft envelope with a piece of packing tape holding the envelope closed & down the middle of the back of the envelope. The envelope was ripped open down the side & the book is missing. I'm going to wait to mark it as anything, because I've had USPS repackage books a couple times before & they eventually arrived so I'm hoping that will be the case here. 

In the mean time I'm wondering if I should message the sender & let them know & if I do that I'd like to kindly let them know that the packaging was insufficient for that size of book? How should I word my message to not offend?

When I receive books that arrive damaged or almost do not arrive due to the packing, i.e., poorly taped, thinly wrapped, loose in envelope, etc., I usually e-mail the sender to let them know what happened and why. I usually receive a thank you from them. Unlike you, I have yet to receive an empty envelope. I had one once that was wide open on a long side. I was surprised it arrived.

Often, these are new members who haven't read how important it is to wrap the book properly.  And instead of sounding off like an expert, I'll mention that I learned the hard way that good wrapping means getting that credit when the book arrives and is in good shape, or that the I learned from the advice of other members who probably learned from experience.

This happened to me a couple of weeks ago.  The sender used PBS postage.  You can go ahead and mark it as "Empty Wrapper Received" now.  It will still be a few weeks until the book will be declared lost in the mail, and you will get your credit back.

It is the sender's responsibility to contact the post office about the lost book, so this would be a good time to get them started on that process, too.  (PBS will explain this to the sender.)

I did not receive any response from the sender, but this is what I said - "Please let me know if you are able to locate the book through USPS.  The package was torn along just one edge; some extra tape on the edges may have helped.  Thank you."

If you mark it appropriately (Empty Wrapper Received), and the book turns up even weeks from now, you can still  go into your Transaction File and mark it appropriately.  That way you're not waiting to get your credit back.  Good luck. Pat

Yes, you want to notify the sender so that, as Betty pointed out, they can fill out the appropriate mail paperwork for lost/stolen/vanished mail.  If the post office finds the book, but doesn't know where it belongs, that can help them get the book to you or back to the sender.