...wakened from a restless sleep with the message from his subconscious still echoing in his brain: The Ramans do everything in threes...
Ironically, Arthur C. Clarke never intended for Rendezvous With Rama to be the start of a trilogy when he wrote that, even though it's an obvious trilogy hook in hindsight. It never occurred to him until readers kept asking when the other two books were coming out.
Even with a smiley face this is still a tiresome and rude response.
The line I quoted is the very last line in the book, and is equal parts fascinating and chilling, a very apt end for a great book.
I highly recommend you read it. If you are in the USA and have a library card, may I suggest the Libby app for Android and iPhone? Or of course buying it second hand from online bookstores is also an option: looks like a new paperback is about $4.55 at smile.amazon.com, used is about 1 to 3 dollars, Kindle version is $7.59. I don't recommend the other books in the series, they don't live up to the original, in my opinion.
A similar book is Eon by Greg Bear, although less pure sci-fi exploration and more sci-fi adventure.