this one is a good book to start with.
Indeed! you had only scratched the Fortran surface, there are a lot of history. As other had advised you, don't waste time with versions before f2003. you need to know that ther are 2 kind of versions, the main as 77, 90, 2003, where the improvments are the main goal, and the complementary or advanced as 95, 2008, 2018...
Also the mixed language. You can always produce modules, subroutines, GUI's in fortran, c, c++, Tcl-Tk, etc, and LINK their *.o with Ld, (in this case of GNU C compiler) to produce your program. It's tricky, but you can do it.
Python has a lot of visual parts, but Fortran had a lot of programs to do graphics, as Dislin, Gnuplot,
Dont waste your time Objectiving everything, OO is good for Java, or C++, even Fortran (since 2003) has facilities to work with, but no thanks, it isn't worth it.
as others had advised you, not only study the fortran code, but follow the Frotran main stars, like Schivers, Chapman, Goldberg, Metcalf, Allan, Reid, etc.