As a teenager in the 50s I really loved Edgar Rice Burroughs' martian series.
Only the 3 first volumes were translated into Danish. They are the best of the very long series.
The first 3 Volumes are:
1. A Princess of Mars
2. The Gods of Mars
3. Warlord of Mars
They are in the public domain.
A beautiful 100th anniversary edition can be found on
Amazon