Hi, James. Overdrive will sell you individual books for a fee (like a bookstore, not a subscription). But if you want free library books, then you have to get them directly through a library website. You can check to see if your MIL has a library nearby at this link.
http://search.overdrive.com/
Free Library of Philadelphia is a popular source for non-residents to purchase access. You might want to check it out. I haven't tried it myself since my local library has a good selection, but it is frequently recommended by others on MobileRead.
http://libwww.freelibrary.org/register/getcard1.cfm
I hope your MIL enjoys her Christmas gift!