What is an Aye-aye as described in the nonfiction book, Last Chance to See?

The Aye-aye is a nocturnal lemur found only on Madagascar. The few Aye-aye's known to exist are found in a refuge on the tiny rain-forest island of Nosy Mangabé. The Aye-aye looks as though it was assembled from bits of other animals. The most extraordinary features are a middle finger that looks like a long, dead twig and its enormous eyes.