Cosmic Engineers

Novel, first published: Astounding Science Fiction, Feb.-April 1939

On Pluto, mysterious messages are being received from the galaxy. Two newspaper reporters, on their way to Pluto to cover the story, stumble on Caroline Adams, a scientist exiled for a thousand years of self-induced suspended animation. Caroline, a spunky female rare in pulp space opera, was declared a traitor for refusing to turn over a discovery that would have won the current war. During her long sleep, she has developed psi powers that enable her to interpret the messages. The messages are from a billion-year-old race, the Cosmic Engineers, who warn that the universe is in danger of collision with a second universe. The Engineers, in communication with the inhabitants of this second universe, have summoned the best minds from every race in the universe to solve the problem. To complicate matters further, the Engineers are at war with the Hellhounds, another evil race capable of navigating the "interspace" between universes and therefore willing to see the Universe destroyed if they can dominate the next one to form.
With the help of the Engineers, the Earthlings travel to the planet of the Engineers at the edge of the universe, where a conference of aliens (all loathsome to the humans) decides that the humans, because of their courage and imagination, must save both universes. The humans are sent millions of years into the future to get the answers from the last old man on Earth. After a number of hairbreadth escapes (once they are kidnapped by a million-year-old collective mind that has gone insane), they steal energy from the fifth dimension, destroy the Hellhounds, and save both umverses.

Ewald, Robert J.: When the Fires Burn High and the Wind is from the North, p.35-36

Jam-packed with ideas - few of which are developed at any length. Just another classic pulp, really - but more clever than plain old adventures in outer space. For example, one of the ideas explored here is whether or not humanity even deserves to win the war for survival.
Another theme is about what it means to be alive, to be sentient, to be sane. As one character says: "Hell, it takes all kinds of people to make a universe." And what it means to be last of a dying or only of an ancient race - compare the two old men our heroes meet...
There are even tossed off ideas that could be developed into whole novellas. A character says "I am damned with an imagination, something no scientist should have. A scientist should just plug along, applying this bit of knowledge to that bit of knowledge to arrive at something new. He should leave the imagination to the philosophers." A couple of sentences later Simak drops that, leaving the reader to work it out for herself.
Another thought I had: Simak truly is a smart guy. Yes this is set improbably far in the future, and the personalities and their challenges are improbably like ours. But after a while a reason for that situation turns up. Not exactly a 'deus ex machina' but a bit of the intermediate history of humanity - but again not drawn out. The focus here is on ideas & action.

Cheryl in CC NV: