I looked for a link from PBS, didn't find it.  Found too much info when I googled it, and when I ask at the post office they pull out a 3-ring binder with faded photocopies in it (I haven't seen the girl I know who works there and tends to be one of the smarter ones, had me send game pieces non-machineable I've wondered if that would be better for books as well?).

What are the rate regulations for the different types of postage.

In specific I am considering posting in the Book Bazaar but need to know what my cut-offs will be for cheaper shipping (aka when it gets cheaper to send bulk vs media etc.)

The Media Mail rates are good up to 70 pounds, first class up through 13 oz.  Where it applies, Media Mail rates beat any other option I'm aware of.

PBS has a Media Mail and First Class chart in the help documents:

Current USPS Media Mail rates:
  Effective March 14, 2010

Up to 1 pound.......................................$2.38
More than 1 pound, less than 2 pounds....... 2.77
More than 2 pounds, less than 3 pounds..... 3.16
More than 3 pounds, less than 4 pounds..... 3.55
More than 4 pounds, less than 5 pounds.....3.94
More than 5 pounds, less than 6 pounds.....4.33
More than 6 pounds, less than 7 pounds.....4.72
More than 7 pounds, less than 8 pounds.....5.11
More than 8 pounds, less than 9 pounds..... 5.50
More than 9 pounds, less than 10 pounds....5.89

Rates in blue are the most common postage rates for single-book shipments.

Current  USPS First-Class Parcel rates:
Effective March 14, 2010

Not over 1 ounce..............................$1.22
Not over 2 ounces..............................1.39
Not over 3 ounces..............................1.56
Not over 4 ounces..............................1.73
Not over 5 ounces..............................1.90
Not over 6 ounces..............................2.07
Not over 7 ounces..............................2.24
Not over 8 ounces..............................2.41
Not over 9 ounces..............................2.58
Not over 10 ounces...........................2.75
Not over 11 ounces............................2.92
Not over 12 ounces............................3.09
Not over 13 ounces............................3.26

If your package weighs 7 ounces or less, First-Class rate will be less expensive than Media Mail rate.

BTW - Media Mail @ 69 pounds would cost $28.90 postage.

Earlier this week I mailed a package of 8 MMPB & 1 tradesize paperback, including the box/packaging total postage was $4.13.

You can also calculate your postage anytime at http://postcalc.usps.gov/

Thank you all, that helps tremendously.

had me send game pieces non-machineable I've wondered if that would be better for books as well?).   No, non-machineable is only for letters.

What are the rate regulations for the different types of postage.

In specific I am considering posting in the Book Bazaar but need to know what my cut-offs will be for cheaper shipping (aka when it gets cheaper to send bulk vs media etc.) do you mean first class vs media? unless you are a large mailer, you cant send bulk rate.

I think the OP was confused about how the various US Mail rates work. At light weights, first class will be cheaper than media mail, but as you get heavier media mail is always the cheapest rate for a member of the public. They might have been confused about the parcel post rate, but that's not ever going to be cheaper than media mail.

For media mail postage totals needed, you weigh your package, subtract 1 pound and multiply by 39c (per pound after the first one) and add $2.38 (for the first pound.  I have been doing this for large book packages I have been sending to APO addresses through "Operation Paperback" and weighty packages containing old LP records, which DO QUALIFY AS MEDIA AMIL, too.    Of course, if your package weighs 3 lb. and 1 oz., you have to use the 4 lb. rate.  The clerks are often amazed when I come in with a package with $9.79 in postage on it weighing 20 lb. and the rate is correct.