I the protagonist had been best friends with Gray Maturin and in love with Isabel Bradley, however the war changed him and he feels the need to see Europe and eventually studies under a sage in India for three years.  
During his time in Germany, the protagonist of this novel is seduced by Ellie and travels around with Kosti.  The entire story is narrated by the real-life author who learns much of it from his friend Elliot Templeton.  Before World War I the protagonist had been best friends with Gray Maturin and in love with Isabel Bradley, however the war changed him and he feels the need to see Europe and eventually studies under a sage in India for three years.  FTP, identify this work about the philosophical wanderings of Larry Darrell, a novel by Somerset Maugham.
During his time in Germany, the protagonist of this novel is seduced by Ellie and travels around with Kosti.  
The Razor's Edge