I'm not 100% sure what to do.  I already marked it on here, but do I also call the Post Office? This has never happened to me before.

Here is the Help Doc which describes what you should do:

http://www.paperbackswap.com/help/help_item.php?id=333

There is a better forum for your question than this one ... the forum called "Questions About PaperbackSwap" is for  "What should I do in this case?' questions.

This forum sticks to questions that are specifically about the USPS, usually. And gets less traffic.

Welcome to PBS!

I sent a book in a standard Mailer and it arrived empty. I called the PO but was unable to get any help in tracking down the book. I just received a mailer that had been opened. Fortunately the book was in the mailer undamaged. From reading other posts I realize that they can open and inspect but it seems like they should reseal the package after they do it.

Welcome to PBS Damien!

Yes, John.  If you received an open package (esp. after inspection) you should go talk to your local postmaster.  Packages should be properly sealed after a postal inspection.

John, yes, they can open packages for inspection, but they DO reseal them, if you got a package that was "opened" it was due to inadequate packaging. most newbies don't understand what adequate packaging means and they just put a book in a standard mailer and seal it up with the glue on the flap. they dont fold the mailer down tight around the book and they don't use 2" packing tape. If the book can move in the packing, it WILL probably rip the packing during processing.

Unless the package is stamped "opened for inspection" and a letter is placed inside detailing the inspection with a pass/fail, the package wasn't inpected, it just broke open during processing. 

wow Damien.  you joined one day, and got a book 5 days later?  over the holidays!  that's impressive!