Whenever I'm working in this area I try to hit up Thrifty Joe's because they always have a very diverse selection of used books. And the prices are generally reasonable too. On my last visit I got the newest Steven Raichlen book in like new condition for less than half the new price, a great Vegas coffee table book, and a handful of other items for less than 40 bucks. It's a nice place to browse when I'm killing time over the lunch hour.
While I was there last week, I did hear that the plaza has new owners so they are going to be making some changes, and Thrifty Joe's and a few other businesses will be moving to other spots in the plaza while some remodeling is done.
Having a post office in the store as well is very convenient, but can also make it very busy. I have never had a good to great experience with the staff, and most seem rather distracted or unwelcoming, but that won't stop me from coming back if I'm in the area again.