I suspect that nearly all of the Fortran code that will exist ten years from now already exists today, so knowing the language is a skill that is more likely to be useful to you for performance testing and code porting than for new development.

The trickiest part of really learning Fortran today is that it is hard to define what the language is, apart from the practical definition imposed by what its seven or so surviving compilers accept and how they interpret it. There are near-universally portable features that are not part of the ISO standard; there are standard features that are not at all portable, or not available at all anywhere. So what one should know as “Fortran” is its reasonably portable intersection of features across multiple compilers, and there isn’t a good practical book that will teach you that.