If you are serious about learning Fortran then you should start with Fortran 77, most Fortran code out there will be written in the 77 standard. The newer standards may have better ways of doing things, but they are not used as much in the real world.
In my department both of our Fortran applications are written in Fortran 77. Our weather satellite application as well as our data feed application, are both \written in Fortran 77. These are just two applications, but I think you will find that a significant portion of the code out there that must be maintained, will be written in Fortran 77.
It may seem odd but the best books on Fortran will be the older ones. The book I learned Fortran from was originally published in 1989, the revision I used mentions the later standards but it only discuses how thees standards differ from the 77 standard.