Even though in has advertiements inside, could i legally mail a phone book and use media-mail?  I have a feeling the answer is no, but I just wanted to check with the postal experts here first.

I am guessing that it is no also, but that is not a definative answer, as I am not a postal worker. But I would love to know why the heck you would want to send a phone book? And hopefully it is not the Manhattan phone book, or equivalent, those things weigh a ton.

I live in Contra Costa County (Bay Area) in California. Until about 10 years ago, my mother did too. Then she moved to another town about 110 miles away. She is the one I would be mailing my local (Contra Costa) phone directory to. 

Well, the way I read the postal regs, you could probably send the local white pages by MM, but not the yellow pages.  The advertising is the difference.

But, how heavy a book are we talking about here?  Generally small town phone books aren't more than a pound or two, and not very thick.  You could probably fit one inside a Priority Mail flat-rate envelope with no problems, and that would cost you under $5.00

Unfortunately, the phone book I want to send is about an inch or two thick.  That is why I am hopeing I can send it MM.

Only an inch or two? 

   Pick up a Flat Rate Priority envelope and see what you can do.  It sometimes amazes me how much they can hold.

Yup, you can easily get a local phone book in one of them.  We got our new local directory yesterday and I just tried it (I do a lot of shipping and have a closet full of supplies) and it fit with no problems at all.  Ours is an inch and a half thick.