Somebody correct me if I'm wrong but Walpola Rahula was Theravadan? In any case, his book "What the Buddha taught" is an excellent starting point for anyone interested in Buddhism in general. And, yes, I'm almost sure he was Theravadan. There are many people from the Theravada branch of Buddhism here that will come up with excellent books but this one, along with Thich Nhat Hanh's "The heart of the Buddha's teaching" are my top recommendations for newbies. But then, Thich Nhat Hanh is zen, not Theravadan.