I live on a street where we have a "walking" postman. Would it be against the regulations for him to pick up a book to be mailed with the PBS postage? I wound't expect him to take 3-4 packages...but what about just one? 

It certainly is not against the regulations.  But some mailpersons, like mine, refuse to pick up any packages.  They don't want to have to carry them until they get back to their truck.   I'd try it and see.

If you have a curbside mailbox and you leave the package in there with the flag up (and correct postage and following the 13 oz rule), and if the carrier has room in his bag, then yes, he is supposed to pick up the Media Mail package. If he doesn't, you can call 1-800-ASK-USPS and have them document the problem.

That information is on the USPS website and I posted a link to it here the other day.

I know a lot of people have been told otherwise by their mail carriers and by their Post Office, which just goes to show that some USPS employees don't know or follow their own rules.

I was told by USPS and had it confirmed several places:  mail carriers are obligated to DELIVER mail, but they are under no obligation whatsoever to pick up the mail from any residence, media mail class or not.  It is totally up to their discretion whether or not they do so, unless the person who has the mail pickup has scheduled Carrier Pickup service with their post office.