At the end of the first book,

And on far-off Earth, Dr. Carlisle Perera had as yet told no one how he had wakened from a restless sleep with the message from his subconscious still echoing in his brain: The Ramans do everything in threes.

This may be an obvious question. I haven't read the two books written by Gentry Lee and only vaguely remember the original series. Please tell me the appropriate book in case it is mentioned.