• http://fortranwiki.org/
• http://fortranwiki.org/fortran/show/Tutorials
• http://fortranwiki.org/fortran/show/Books

The differences between Fortran 90 and Fortran 2003 (or newer) are not that large, so tutorials on Fortran 90 should still be a good starting point.

• https://www.uv.es/dogarcar/man/IntrFortran90.pdf
• http://www-eio.upc.es/lceio/manuals/Fortran95-manual.pdf
• http://www.egr.unlv.edu/~ed/fortran

• “Modern Fortran Explained” by Michael Metcalf. This is a good reference, not a book to start with if you're new to programming.

To quickly look up the definition of built-in functions and procedures: https://gcc.gnu.org/onlinedocs/gfortran/Intrinsic-Procedures.html#Intrinsic-Procedures