Book of Ra is a terribly classic, conservative slot that has become a template for many other slot machines. See for yourself: 5 game reels with three lines of symbols. Today, such mechanics are embedded in most of the modern slots. And there is nothing surprising in this: it is extremely simple and works perfectly.
When 3-5 golden books fall out, the bonus game begins. It is no different from the usual one, with the only difference that the next 10 spins will be completely free.