I just received another book package with no postal info on it at all.  The package had been mailed with stamps in the total amount of $2.61, but the only sign that the PO touched it is a black marker line drawn through the stamps so they can't be re-used. Why is there no date stamp used?

I received a book today which also didn't have any postmarks, or any marks, on the stamps. What really bewilders me is the stamps only total $1.25 and the book weighs 9.6 ounces. 

It is rare that I receive a book with stamps that has been cancelled, more likely either marker over the stamps or nothing.

Like Thomas, I have too received books with not enough postage for the weight and I am always surprised that they don't come postage due. I also receive books weighing, say 8 oz, with postage for a 3 lb book. The weights on PBS are wrong at least 30% of the time, I would never mail a book from home without using scales!

I've shipped almost 1,300 books as a member. Sometimes I use the old-style lick-em stamps, from my collection, to help make up the required postage. Twice I've had to deal with these stamps coming off the packages. Once I had to take the problem to the PO, and another time the member receiving it paid the postage due and told me she didn't mind doing so.

Now when I use the 'link-em' stamps I tape them along their edges to secure them, but still leave most of the stamps open to receive the postal cancellations.

IME not all stamped mail is postmarked these days.  Many of mine are, but maybe 20 or 30% are not.

Out of a sampling of 4 mailers with stamps I got from SwapaCD,  only 1 has the round postage cancellation ink stamp.  I think the PO has abandoned this process.  If you ever go to the PO and see a clerk ink stamping with gusto, there's someone who holds on to the old ways.  

I rarely see a cancelation and actually in my post office while I'm standing in line I see people who want cancelations ask to have it done and the clerk stamps it there.

I never read them so don't know one way or the other.  If I receive the book, it's all good.