Letters on absolute parallelism (1929-1932) Robert Devever, Elie Cartan and Albert Einstein (1979) (233p) [Inua].djvu

Title: Elie Cartan - Albert Einstein: Letters on Absolute Parallelism, 1929-1932
Editor: Robert Debever
Hardcover: 254 pages
Publisher: Princeton University Press; First Edition edition (October 21, 1979)
Language: English
ISBN-10: 0691082294
ISBN-13: 978-0691082295

The problem discussed mainly seems to be: Rij=0
which if Rij refers to the Ricci curvature tensor is or is very like the null Ricci problem that resulted in modern Calibi-Yau theory. They seems to respect each other, but not their different approaches to the mathematics involved.
This book is about what parallel means in terms of tensors and functions inside a spherical manifold.
More interesting than the context is the subtext of two of the most important men of the 20th century exchanging nearly polite letters like this and having them collected and published.