Bursting with nervous energy, he told me about his business travails — work so busy he was staying regularly until 10 at night, and a billionaire client sapping his energy and causing him grief.
bursting with nervous energy는 분사구문인 것 알겠습니다.
work so busy he was ~에서
work so busy가 어떻게 오게 된 것인지 잘 모르겠습니다..
앞의 his business travails 와 동격으로 work 을 썼구요 뒤에 which was 가 생략되었습니다 다시 busy 뒤에서 that 이 생략되었구요 그 댓은 so 형용사부사 that 절에서의 생략입니다 and 뒤에서 a billionaire client 다음에 was 라는 비동사가 생략된 구조입니다