I have a Bachelor's degree in physics and I enjoy advanced calculus but my real interest goes to logarithmic/ polylogarithmic integrals and harmonic series.
I wrote a book about the harmonic series
(An Introduction To The Harmonic Series And Logarithmic Integrals) and it's available on Amazon https://www.amazon.com/dp/1736736027
The eBook PDF can be found for free on the website https://www.harmonicnumber.com