I hadn't received a book that was ordered some time ago--I received an email from Paperback Swap that it was being marked lost in the mail, and that my credit had been refunded.

The sender of the book contacted me this morning and said that she had forgotten to actually send the book, and had sent it out in the mail today. This is fine with me, albeit a little irritating.

When it actually arrives, I just need to go into the Transaction History and mark it as being received, correct? At that point will the credit be deducted from me, or does the sender forfeit the credit for taking so long to mail the book?

the credit will be deducted as soon as you mark it Received.

When you get the survey upon receiving, include the date the book was mailed.  A record will be kept of her late-mailing and she'll receive an automatic email about it.

What Debbie said..., and also, the book will have been placed on your reminder list (if there were other copies available) or your wish list (if there were no other copies posted) when it was declared lost, so be sure to delete it from the list. 

And some info from the Help Center that you can share with the sender if you choose:

If you marked the book mailed but did not mail it and you want to mail it late (more than 4 days after the date you marked it as mailed),  you MUST contact the requestor using the PM button on the transaction, to be sure that the requestor still wants the book.  (If the book has already been declared "lost", the PM button is on the "lost in mail" transaction in your Transaction Archive.)

• If she replies that she does still want it you can mail it to her.  She can mark it received and you will get credit at that time.
  • If the book has already been declared "lost" she can mark it received from her Transaction Archive and you will get credit at that time.
• If she replies that she does NOT want it (she may have gotten a copy from someone else in the meantime), you should NOT MAIL - you should ask us to cancel the "en route" transaction and you can repost the book for someone else to request.
• If she does not respond to your PM, you should repost the book.  It will be offered to her automatically if she still wants a copy.
• You should not mail a "lost" book to the original requestor unless she has confirmed that she still wants it in a Personal Message.  If you mail it and she has already gotten another copy, she will NOT be obligated to mark the book received, or to return it to you.

It if went on your WL you might want to put that book on hold.  Then if the book doesn't show up in a reasonable amount of time you will keep your place in line incase other add it to their WL. But you won't have to decide to accept a WL offer for it if it's posted again quickly. 

Good point Mary, I was thinking she'd already received it.