Comments under the Space SE question How do orbits around Jacobi ellipsoids behave? include:
Periodic orbits around a rotating ellipsoid "This paper extends results obtained during the completion of the author’s doctoral dissertation under the supervision of Jürgen Moser. (Kammeyer, P. C. 1974, Thesis, New York University)" which would probably make for excellent reading!
..and Carl Ludwig Siegel's "best student was Jürgen Moser, one of the founders of KAM theory (Kolmogorov–Arnold–Moser), which lies at the foundations of chaos theory."
Wikipedia's Jürgen Moser; work says:
Moser completed his undergraduate education at and received his Dr. rer. nat. from the University of Göttingen in 1952, studying under Franz Rellich. After his thesis, he came under the influence of Carl Ludwig Siegel, with whom he coauthored the second and considerably expanded English language edition of a monography on celestial mechanics. Having spent the year 1953 at the Courant Institute of New York University as a Fulbright scholar, he emigrated to the United States in 1955 becoming a citizen in 1959.
Question: What monograph on celestial mechanics did Jürgen Moser coauthor the 2nd and considerably expanded English language edition of with Carl Ludwig Siegel?