It masters the Einstein Summation Convention, which is often the biggest hurdle for new learners.
This is the gateway to general relativity. Chaki’s derivation of the Riemann tensor from the commutator of covariant derivatives is particularly clear. tensor calculus m.c. chaki pdf
If you download a free PDF, cross-check page numbers with a library copy. Ensure that the section on “Ricci’s theorem” and “Bianchi’s identities” is fully readable. It masters the Einstein Summation Convention, which is