Binary file layout reference

Where are good sources of information on binary file layout structures?

If I wanted to pull in a BTrieve index file, parse MP3 headers, etc... where does one get reliable information?

I'm not sure if there's a general information source for this kind of information. I always just search on google or wikipedia for that particular file type. The binary file layout structure information should be included.

For example, MP3 file layout structure

Not comprehensive, but useful nonetheless: http://www.wotsit.org/