what is the cheapest way to package and mail these?

probably a padded envelope and then of course media mail.  unless it is over 2 lbs (and not many hardbacks are) it would be $2.47 to mail

I wrap with saran wrap and then tape several sheets of typing paper together and then wrap like a present.  I reinforce the edge with packing tape.  I send Media Mail and most are $2.47

I take a piece of corrigated cardboard, cut a little longer than the highth of the book and long enough to wrap all the way around. I tape it VERY well. I run the tape all the way around the book several times. When its done you can see the pages from the ends of the package(so USPS employees can see that it is only a book). I have shipped several books this way and had no complaints ( and some great compliments)

I find padded envelopes, and then sending media mail is cheapest.

I have never had to send more than one hardback though, so I don't know if it gets more expensive.

I use the brown paper bags from the supermarket.  I send out a lot of hardcovers this way and have never had a problem. And the bags don't cost anything.