I don't know if this is what you're looking for but it may help others pinpoint the book. The book I'm remembering has aliens seeking a new home, knowing that their home world (in a binary system?) will shortly become unlivable. Their ships are powered by null point energy. One expedition lands on earth in prehistoric times, as does a follow up expedition, but they are eventually wiped out by the natives and possibly disease also. (There was something about a dietary deficiency.) In modern times the US and China find the aliens' 'library' roughly the size of a shipping crate on the moon, but the Chinese fear that the US will monopolize the discovery leads to the device being demolished. Then a second such device is discovered elsewhere in the solar system and recovered successfully. There was a subplot about racial equality in that the aliens considered their own servants from a less advanced continent on their world to be a lesser species when in fact they were all the same species.