The most precise specification is the Fortran 95 standard. An official copy is available from ISO or ANSI (webstore.ansi.org) as 1539-1-1997; a draft which is essentially the same as the official copy is available free at http://www.j3-fortran.org/doc/year/97/97-007r2/pdf/.

Another more readable specification is a book "Fortran 90/95 Explained" by Metcalf et al.