As mentioned in a footnote in the blog post, I think Gelfand and Fomin’s “Calculus of Variations” is very clear on its topic. Wald's "General Relativity" is slow-going, but excellent. No good textbooks on quantum mechanics or QFT exist; Weinberg's "Quantum Theory of Fields" is probably the least terrible on that. Nielsen and Chuang's "Quantum Computation and Quantum Information" is still excellent, but pretty out-of-date.

Also, to be clear, I haven't worked through most of Sussman, so I can't recommend it one way or the other. I was just commenting on the handling of the Legendre transform.