The following free books and resources are useful starting points for learning Fortran 77:
Page, Clive G., Professional Programmer’s Guide to Fortran77 (PDF, 493K), 22 February 1995.
Chivers, Ian D., and Jane Sleightholme, Interactive Fortran 77: A Hands on Approach, Second Edition, 1990.
This one has a quote from the Hitchhiker’s Guide on page 1 so it can’t be all bad. The link is to a PDF file.
Press, William H. et. al, Numerical Recipes in Fortran 77, Second Edition. 1992.
This one is pretty standard and well-known, although it’s more for people who already know Fortran and have a specific numerical problem to tackle.
The Fortran FAQ, 3 January 1997.