Original: There he acquired a copy of Die Rakete zu den PlanetenrÃ umen (1929) (By Rocket into Interplanetary Space) (in German) by rocket pioneer Hermann Oberth.
Simplified: He bought with his extra money the book Die Rakete zu den PlanetenrÃ umen (English: The Rocket Into Interplanetary Space), which is the basis of the modern rocket research.