Might be wrong, but I think you'll find yourself paying to use any library in Wake County unless you're a county tax payer, seems some years ago I was told by someone that they tried checking a book out in Garner and because they didn't live in Wake County they'd have to pay $15.00 to receive a library card as a fee for out-of-county user fee.
Non-Wake County residents can get a Wake County library card for $25: http://askwcpl.wakegov.com/a.php?qid=53736
Harnett County will give its neighboring counties free library cards. They even have free movies. That is unless something changed in the last 2 years.