Signals seem to be one of those areas that should be conceptually simple and easy to explain but I have never come across one source that is both comprehensive, lucidly written, and up to date. In part this seems to be because of historical cruft, lots of exceptions to rules, different programming standards, the confusion threads throw into the mix, real-time additions, etc.
What's your best written, most complete source on unix/linux/posix signals? Web sites, books, stone tablets, whatever, are acceptable.