There's a cool store with only books by or related to Kafka, if you're interested in him at all. It's somewhere near the Old Town Square. Most people speak English as well as Czech in Prague. As long as you learn please and thank you, you'll be fine (thank you isn't pronounced as it's spelled, by the way, so ask someone how to say it before attempting it or you may be really off).
Astronomical Clock when chimed is astronomically disappointing but beautiful to look at!