The second of the linked books is Mission To The Heart Stars. In it the same crew, Jack Loftus et al, are sent on a journey to the center of the galaxy. There exists a federation, The Heart Stars, of advanced and stable civilizations. The Hegemony of Malis consists of million year old civilizations who have been watching Earth, along with other planets.
Earth has 50,000 years to measure up or else. And the Hegemony is ruthless enough and powerful enough to enforce their rules. Mars was being observed when they decided to commit racial suicide and the Hegemony just let them.
But the earth has a hole card, its alliance with the Angels.
Again I have a divided opinion on this novel. The alien civilizations that are in the book are interesting. When this was written the existence of aliens superior to Earthlings in SF stories was rare. John Campbell was not able to enforce his edict against superior aliens everywhere, but many writers followed it anyway.
Blish also seems to come down more on the side of individual liberty being important than in the first book The Star Dwellers. But this is mainly in contrast to the tyrannical political system of the Hegemony. There are still to many things about this future society that make me uneasy.
He also writes in the foreword, and in the novel, about a High Energy Civilization. This was a concept that was prevalent at the time. SF writers who were often fans of Atomic Power would talk about electricity too cheap to meter.
And if we ever get to that state it will change our society drastically. But Blish never foresaw the oil crisis, and the problems that developed with Atomic Energy.
In these books its actually some form of fusion that exists and that potential is still with us. But its been many years in development and seems no closer.
Neither of these books are as good as the first two- Welcome To Mars and The Vanished Jet, but they are worth a read.
Next up, a juvenile by a lesser known writer, and the only thing by him I have read I believe.