How does it work if I request a book and it's marked as mailed but it's never received? Do I get a credit back and get my place back in line? I'm guess I'll probably get a credit right but not my place in line?

If the book is wish listed, you get your credit back AND are put back at the top of the list.  PBS does encourage you to put the book on hold to give it another week or two.

I believe the book is put on your reminder list if it is not WL.

From help:

If a book you requested does not arrive by the 26th day (the 35th day if the sender or requestor is outside the contiguous US) after it was marked mailed, it will be declared lost in the mail at PBS. The requestor does not mark the book Lost; the system does this. 

The requestor will get credit back, and the book will go onto the Wish List (if there are no available copies in the system) or the Reminder List (if there are other copies available to request).  If the book was a Wish List book when it was requested before, the requestor will go back to his or her previous place "in line" for the book.

• However, this does not mean that the book is definitely lost.  Sometimes it is merely delayed en route.
• If the book arrives after it has been declared Lost, the requestor must mark it received from the Transaction Archive.
  • At this time, the credit will be taken back and given to the sender for sending the book.

What you should do if a book you requested is declared Lost in the Mail:

• Contact the sender, to find out if mailing was delayed for some reason.
  • you can do this from your Transaction Archive, using the PM button on this Lost transaction in that list.

• If the mailing was not delayed, the sender should call 800-ASK-USPS or go to http://postalinspectors.uspis.gov/forms/MLNtRcvd.aspx (the Postal Inspection Service's website) and input the information online there . This will help USPS try to recover the book if it truly has been lost in the mail.
• Don't re-request the book for at least 2 weeks, unless you are willing to run the risk of getting two copies (and paying a credit for both).

I usually wait the two weeks before reordering (if it's not on my WL).  When PBS marks it lost in the mail, I will also send a PM to the sender letting them know I didn't receive it and when (if) it's received I'll be sure and mark it appropriately.  I've had senders respond that they forgot to mail the book, etc (even though they did use PBS postage).  In that way, I can have some idea also whether to reorder or to wait.  Pat

Thanks guys!