There's also Fundamentals of Astrodynamics https://www.amazon.com/Fundamentals-Astrodynamics-Second-Dov...

Which builds up from first principles a ballistic missile defence system. Always useful to have.

This type of learning material, lacking any desperate 'ELI5' oversimplifications and laying out the proper amount definitions and equations without having to resort to fancy animations to keep the -quite often indolent- reader engaged, is unfortunately scarce in today's web, as is evident from the website's layout.

Here are two favorite readings for anyone willing to delve more into the subject:

    Orbital Mechanics for Engineering Students by Howard D. Curtis

https://www.amazon.com/Orbital-Mechanics-Engineering-Student...

    Fundamentals of Astrodynamics (aka BMW) by  Roger R. Bate, Donald D. Mueller, Jerry E. White, William W. Saylor

https://www.amazon.com/Fundamentals-Astrodynamics-Second-Dov...

Another great textbook as suggested by musgravepeter in the comments is

    Fundamentals of Astrodynamics and Applications by David A. Vallado, Wayne D. McClain

https://www.amazon.com/Fundamentals-Astrodynamics-Applicatio...