I have two books en-route to me from two different locations. Both, according to PBS tracking should be at my post office by now but both are still showing (the little postal truck) as being not quite there yet. Its quickly approaching the cut off date for lost in the mail. What could be up with these books?

I'm in Chicago, and we got 20 inches of snow the Sunday before last, leading to me getting mail only three days last week. The East Coast, especially Boston, also has gotten slammed with snow. It's possible mail, especially Media Mail, got slowed down in many places for the last little bit.

Sometimes books take crazy routes. I had one go from north of me by about 10 miles out to Iowa and then back to me!  

That was the situation in Hartford also.  My mother received 10 books yesterday that had been held up at the local Post Office for 4 days because of the snow.

As already mentioned, the little blue truck is best used for entertainment purposes only....it's been known to drive around in the ocean and take other scenic routes.  Look under the 'Details' button for actual USPS scans.  The delay may be weather related (note if your book was recently scanned in one of the winter storm sorting centers), or sometimes they are merely incorrectly sorted and travel way out of the way before being re-directed to you. 

But often, the package is just moving slowly...Media Mail rates are based on 'as space available' status.  If the bin/truck/etc is full, this mail rate may wait for space.  Sometimes there is room in the First Class bin and your package travels across the entire country in just a day or two!