There was a book I read 28 years ago. Remembering it was a good read I just want to re-read it, but I don't remember its name.
The synopsis is (as I remember):
Mankind is about to go to the Moon for the first time, and after they get there they find traces of civilization, with still-working machines. They find a lot of clues which lead them to think that aliens who lived there are still located on the Mars. They build one communication device and try to communicate with aliens on Mars, and they respond. With plans given from Mars they build some stronger rocket, and the main protagonist flies to Mars also. He finds there that the "aliens" look just like common people, although they are taller a bit.
And there was also a third species in the solar system, I think on Ganymede. They were 20 feet tall, called themselves "Lothar" or something similar. They had some kind of taboo about flying to the inner part of the solar system, so they never actually made real contact with Mars people, although they were at war. They live in globe-shaped houses that levitate.
There is a mention that war between two races was ended by destroying the planet between Mars and Jupiter, where now an asteroid belt can be found.
They mention a fourth civilization also, but nobody knows anything about them, except that their technology is superior.
I tried to search for it on the internet, but I can't seem to find any information about it.
I'm not sure that the story I wrote above is 100% accurate, I was a child then. I remember the book was very thick, having lots of pages.
Does anybody know what the title is and who is the writer?