For some context surrounding the mathematical difficulties of general relativity at the time, I recommend the book Einstein's Italian Mathematicians: Ricci, Lev

For some context surrounding the mathematical difficulties of general relativity at the time, I recommend the book Einstein's Italian Mathematicians: Ricci, Levi-Civita, and the Birth of General Relativity. https://www.amazon.com/Einsteins-Italian-Mathematicians-Levi-civita-Relativity/dp/1470428466/ https://www.amazon.com/Einsteins-Italian-Mathematicians-Levi...