When you're down...
For me I think Agatha Christie or P.D. James are the main authors I turn to. There's something very comforting about detective fiction - it's rather like the adult version of the fairy story. Terrible things may happen, but in the end wrongs are righted, order is restored, and they all live happily ever after (except for the victim, and usually the murderer).
Then there's the odd book that has been with me through most of my life - it may be dated but One pair of hands by Monica Dickens never fails to make me laugh, as does The wind on the moon by Eric Linklater - one of the best kids' books ever, or books that take you into a different world (whether real or not) such as Dodie Smith's books, or wonderful travel writing such as Peter Fleming's News from Tartary.
Perhaps as much as anything it's the comfort of reading something familiar when you're not coping with life very well - you may feel as though life is collapsing around you - but Cadpig will still be rescued by Pongo, a tin tack will still be left on the butler's chair, and death will still happen on the Nile - and thank God for it!