This week's book giveaway is in the OCAJP 8 forum. We're giving away four copies of OCA Java SE 8 Programmer I Study Guide and have Edward Finegan & Robert Liguori on-line! See this thread for details.
The Javadocs for WeakHashMap explain when it's appropriate to use - e.g. when you're using keys of a class that overrides the equals() method you can get unexpected behaviour. And that probably covers the majority of uses of Maps. Of those cases where it would be appropriate, it often won't make much difference.