Human capital theory

https://ocw.mit.edu/courses/economics/14-03-microeconomic-theory-and-public-policy-fall-2016/lecture-notes/MIT14_03F16_lec20.pdf

This may be a reach asking on this platform, however does anyone know which textbook may contain or expand upon the derivation in chapter 2 of the linked notes? Essentially I was reading up on Human Capital Theory and I came upon that derivation which confused me as it didn’t seem directly related to HCT. Could someone explain how the result actually relates to HCT? Specifically, the implications of the statement: "the wage increment for one more year of schooling must be approximately equal to the interest rate" (top of pg5). Similarly: "the estimated rate of return to a year of schooling has been about 5 to 10 percent—approximately equal to the real rate of interest plus inflation."

Even if that were true – empirically or what not – what’s the link to human capital theory exactly? In other words, what part of the derivation proves implication (1) and (2)?