Answer based on context:

Prior to creating the Asterix series, Goscinny and Uderzo had previously had success with their series Oumpah-pah, which was published in Tintin (magazine) magazine. Astérix was originally serialised in Pilote magazine, debuting in the first issue on 29 October 1959. In 1961 the first book was put together, titled Asterix the Gaul. From then on, books were released generally on a yearly basis. Their success was exponential; the first book sold 6,000 copies in its year of publication; a year later, the second sold 20,000. In 1963, the third sold 40,000; the fourth, released in 1964, sold 150,000. A year later, the fifth sold 300,000; 1966s Asterix and the Big Fight sold 400,000 upon initial publication. The ninth Asterix volume, when first released in 1967, sold 1.2 million copies in two days.

WHat year was the second book published?