For those who already read Rendezvous with Rama but need their alien aliens fix I can highly recommend "Shroud" by Adrian Tchaikovsky. It is a similar theme with modern writing and convincing aliens as is pretty much expected by now from Tchaikovsky.
I liked shroud a lot, but the ending felt very ungratifying. It's like Tchaikovsky wrote himself into a corner and didn't really know how to wrap it up nicely. I find this to be true of some of his other books as well.
I just read a series of his books involving "unspace" so nice to see a recommendation. Will check out Shroud.
Tchaikovsky has written some of the best books I have ever read like Children of Time and Children of Ruin and also some of the worst books I have ever read like Cage of Souls.
Children of Time was excellent, so thank for this recommendation.