(And yes, I know I will be informed of my ignorance very soon after this blog post has been uploaded. People will send me a list of literary works with 'squirrels' and 'secrets' in them. Go on.)
ADDED in December 2011: "If we had a keen vision and feeling of all ordinary human life, it would be like hearing the grass grow and the squirrel's heart beat, and we should die of that roar which lies on the other side of silence" (George Eliot's Middlemarch, chap. 20).