Because I write my own addresses for the pkgs. I mail; I walk into the P.O. and they are stamped and postmarked right there in front of me. Is this the same for PBS wrapped and mailed pkgs? A book I received was marked by the member as mailed on 8/16 , but the date on the stamp reads Aug.26,  10 . So did the member mail late or did the P.O. let it sit for 10 days before stamping it?

It's impossible to know. It could have been dropped off on the 16th and then just before being delievered to you a postal worker noticed the stamps hadn't been canceled so they did that on the 26th.  When did you get it? Where was it coming from? Today is only 8/28, so if it was actually mailed on 8/26 that's a pretty quick trip no matter what!

From PA to VA a real quick trip, I see now it was mailed First Class.. I always wait till I get home from the post office before I mark it mailed in the system; just in case I get way-layed by life or ice cream

If it was PA to VA first class, she probably did wait until 8/26 to mail it. I would note that when you mark the book received. If it was a mistake, it won't hurt her any, I think you have to have a lot of late mailings for it to be an issue.

Yes, that is when the person mailed it.  Look at the postmark and see if you can read the city of cancellation.  It should be from the state from the person who mailed it to you.  In this case PA.  If stamps are not postmarked when being delivered, most times a line is drawn through the stamps, and not a postmark or round date stamp.  For first class two day delivery is about right as well as three day delivery.  Of course sometimes a few take longer.   

We hear rumors that postmarks/cancellation can be applied anywhere enroute.  As Priscilla mentions, you can look for an actual city on the postmark stamp...but I can rarely read the date on those, much less additional information.  The most accurate indicator of actual mailing date is often the date printed on counter postage.