When I began the creation of a new front end the only source of information was an article 'Writing a Compiler Front End' from 'Using Maintaining and Enhancing Cobol for the GNU Compiler Collection' by Tim Josling.
Details regarding GCC can be obtained from the home page of GCC. Information was also obtained from the GCC manual that is available with each distribution of GCC.
But the vital source of information is the front ends of other languages. But I'll never recommend them for the beginner. Once you are on track then they will be of great help to you.