I just remembered this one:
Whenever I'm really sick (especially if I have a migraine), or really sad, or really stressed out and it's going to take more than a hug to cheer me up or relax me, we bring out the big guns:
A. A. Milne's Now We Are Six.
And I have to read about half the poems out loud. (Sometimes I let my sister read them out loud, if she's around.) And I always read the same ones, in the same order. ("And round about. And round about. And round about I go.")
(Also, I turn to the last two pages of When We Were Very Young, and sing "Vespers." "Hush! Hush! Whisper who dares, Christopher Robin is saying his prayers.")
It never fails to cheer me up. (But I can't read it too frequently, or else I would decrease its effectiveness.)