I don't know about you, but I always find it really easy to buy for my mom. She loves any of the Sanctuary stuff and there's always a piece of jewelry that she's got her eye on. It's also rare to find her without her Kindle Fire HD in her hand (although she uses it for games and Facebook rather than for reading!)
Hopefully if you're stuck for ideas for your mom, this little gift guide will help you a bit. My mom's present was actually the first one I bought back in November.