Becker's is a place that rewards diligence and patience, and above all, having a sufficient block of time at your disposal. Expect to dig while you're there, and I don't mean that metaphorically. You will be digging through heaps of unordered, unalphabetized books loosely grouped only by subject or genre, crammed most literally into whatever space is available, whether sideways, spine up, or stacked on top of other items. If you've ever visited a record store like the old Bill's Records in Dallas, Born Late Records in Fort Worth, or any similarly barely organized mom and pop DIY establishment, then you'll know what to expect here.
The Texana section was my primary area of interest, and it's the main reason I'm recommending this place as being worth a stop. Naturally, local history makes up the bulk of what's available at Becker's, but selections covering other regions of the state are peppered throughout, including boxes full of issues of the Southwestern Historical Quarterly which I found stacked on the floor. If you're a student of state history and the kind of person who enjoys a bit of retail archaeology, and you find yourself in H-Town with time to spare, you might consider paying this second-generation bookstore a vist. Parking is very limited, so you should try to avoid peak periods.
Happy hunting!