I have a book on the way with DC.  When I check it, it says "Arrival at Unit" in my city.  I have never seen that notation before.  Does that that mean it was delivered, or just that it made it to the local post office, or what?

(I have not actually received the parcel).

I believe it means it made it to your post office and was scanned in there.  Likely you will receive it on the next delivery day if not the same day it arrives at the post office. 

Arrival at Unit is the down and dirty way of scanning a piece as "accepted" at your local post office. It is a much faster way than scanning it "accepted" and is often used when there are a lot of scans to do. Like a business dropping off about 50 certified letters with postage already on them.  It's a quick way to get them into the system. "Accepted" takes about 4 times as many entries on the scanner.

At the other end it means "Here and ready to be delivered"

"oops" misread the original post.  LOL

At my post office, we scan everything coming in to be delivered as arrival at unit. It means just that, it has arrived and we will be sending it out to be delivered. My office is kind of small, all packages that arrive early in the morning go out the same day.