Complete description in German of Fortran 90. The book can be used as a textbook but it is mainly for reference use. I find it rather easy to read. It treats and explains everything. It is very similar to the book of Adams et al.
Further information is available.
The official standard for Fortran 90. Rather difficult as a dictionary. Requires that you have read a textbook on Fortran 90. The book is recommended. The complete text is available electronically, see further information on Bo Einarsson's Fortran page.