Complex sentence: His first book, "Pan-Europa", was published in 1923, and contained a membership form for the Pan-Europa movement which held its first Congress in Vienna in 1926.
Split into simpler sentences: His first book, titled "Pan-Europa" was published in 1923, and contained a membership form for the Pan-Europa movement. Coudenhove-Kalergi's movement held its first Congress in Vienna in 1926.