For me, every year, I find Dad is the hardest to pick out gifts for. For Mom and my brother I usually get books (or occasionally films), but Dad's eyesight is so bad that he can't read recreationally anymore (he used to love to).
Audiobooks are more viable now, though those run a lot pricier.
Have you considered getting your dad an MP3 player so you can download audiobooks from someplace like Audible.com?
My hubby is difficult to buy for because he buys stuff he's interested in throughout the year. This year, I've bought him an iPad because he'd talked himself out of getting one. But every other gadget, he already has.
My parents, as a pair, are hard because they're old and don't actually need to accumulate much else. It's a challenge to get them something useful that's not going to take up space in their little house.