A book I requested is taking over two weeks to reach me, and I wouldn't worry about it except that it does have delivery confirmation, it's been in the city I live in for a few days, and I can't make heads nor tails what their wording means. Anyone have a clue? 

• 1/21/2011 6:56 PM - Enroute - DETROIT,MI
• 1/22/2011 7:56 AM - Arrival at Unit - WESTLAND,MI
• 1/22/2011 9:13 AM - Unknown-PC - WESTLAND,MI
• 1/22/2011 9:23 AM - Unknown-OF - WESTLAND,MI
• 1/22/2011 11:23 PM - Unknown-DX - WESTLAND,MI
• 1/24/2011 2:36 AM - Forwarded - WESTLAND,MI
•  

When the mail arrives at my little post office, I scan all the parcels "Arrived At Unit."  I have to scan them all, even if they are not destined for delivery from my office.  So that's your first scan "Arrived At Unit."

Then I sort the mail to the different routes and the box section.  When the mail is all sorted, I scan a barcode at the distribution case, and this generates an automatic scan event for all the things that I scanned Arrived At Unit.  This scan, where I don't scan each separate barcode, just the one at the case, is Distribution Complete.  So that's your second scan:  PC, or Processing Complete.

Then I go over to the Post Office Box section, and start putting the mail in the boxes.  By the time I am done putting the mail in the boxes, the carriers usually have their mail nearly all cased, and are getting ready to pull it down, strap it out, and leave for their routes.  When I am done boxing the mail, I scan a barcode that is posted in the box section, and this generates an automatic scan event for all the things I scanned previously as Arrived At Unit.  This is the third scan:  Out For Delivery (OF). 

I don't know what that "DX" scan is.  Does your post office have more than one zip code?  When I looked it up, it looked like Westland had 2 different zip codes.

That would explain the last scan, "Forwarded".  That would be a separate scan of just that mailpiece, and means that the piece is supposed to be delivered from another unit, station, or branch, or different post office altogether.
 

Your package arrived at the unit, was marked out for delivery, then acquired an unfamiliar DX tag.  Now it appears to have been forwarded to somewhere.  Call your PO and ask.